refactor(agents): replace broken ZWSP sort prefixes with leading ASCII spaces

The ZWSP-based core agent sort prefixes silently failed to produce the
canonical sisyphus -> hephaestus -> prometheus -> atlas order.

Empirical testing of OpenCode's Agent.list() sort behavior shows that
Unicode collation treats zero-width characters as ignorable at the primary
level, so ZWSP-prefixed names sorted alphabetically with non-core agents
interleaved (e.g. Sisyphus, athena, Atlas, explore, Hephaestus, ...).

This commit replaces the ZWSP prefixes with leading ASCII spaces in
descending lengths (sisyphus=4, hephaestus=3, prometheus=2, atlas=1).
ASCII spaces sort reliably before alphabetic characters in localeCompare
under all locales and render correctly in every terminal.

Changes:
- AGENT_LIST_SORT_PREFIXES: ZWSP -> leading spaces (4-3-2-1 descending)
- stripAgentListSortPrefix: now strips both legacy ZWSP and new leading
  whitespace, preserving backward compatibility with existing sessions
- normalizeStoredAgentName / normalizeRegisteredAgentName: extract a
  shared stripSortPrefix helper that handles both prefix formats
- agent-config-handler: resolve user-provided default_agent display
  names through getAgentConfigKey before applying the runtime prefix,
  so configs like default_agent="Hephaestus - Deep Agent" are normalized
- agent-runtime-name-sort.test.ts: new regression test simulating
  OpenCode's exact sortBy logic (default_agent desc + name asc localeCompare)
  to verify canonical core agent order under randomised input permutations
- AGENTS.md: document the empirical finding that ZWSP was broken, why
  ASCII spaces work, and the descending prefix-length contract

Existing strip functions retain ZWSP support so legacy session state and
configs continue to resolve correctly without migration.

src/shared/agent-display-names.test.ts

@@ -194,11 +194,11 @@ describe("getAgentConfigKey", () => {
 })
 
 describe("getAgentListDisplayName", () => {
-  it("applies invisible stable-sort prefixes to the core agent list", () => {
-    expect(getAgentListDisplayName("sisyphus")).toBe("\u200BSisyphus - Ultraworker")
-    expect(getAgentListDisplayName("hephaestus")).toBe("\u200B\u200BHephaestus - Deep Agent")
-    expect(getAgentListDisplayName("prometheus")).toBe("\u200B\u200B\u200BPrometheus - Plan Builder")
-    expect(getAgentListDisplayName("atlas")).toBe("\u200B\u200B\u200B\u200BAtlas - Plan Executor")
+  it("applies leading-space stable-sort prefixes so OpenCode localeCompare yields canonical order", () => {
+    expect(getAgentListDisplayName("sisyphus")).toBe("    Sisyphus - Ultraworker")
+    expect(getAgentListDisplayName("hephaestus")).toBe("   Hephaestus - Deep Agent")
+    expect(getAgentListDisplayName("prometheus")).toBe("  Prometheus - Plan Builder")
+    expect(getAgentListDisplayName("atlas")).toBe(" Atlas - Plan Executor")
   })
 
   it("keeps non-core agents unprefixed for list display", () => {

src/shared/agent-display-names.ts

@@ -27,10 +27,10 @@ export const AGENT_DISPLAY_NAMES: Record<string, string> = {
 }
 
 const AGENT_LIST_SORT_PREFIXES: Record<string, string> = {
-  sisyphus: "\u200B",
-  hephaestus: "\u200B\u200B",
-  prometheus: "\u200B\u200B\u200B",
-  atlas: "\u200B\u200B\u200B\u200B",
+  sisyphus: "    ",
+  hephaestus: "   ",
+  prometheus: "  ",
+  atlas: " ",
 }
 
 const INVISIBLE_AGENT_CHARACTERS_REGEX = /[\u200B\u200C\u200D\uFEFF]/g
@@ -40,7 +40,7 @@ export function stripInvisibleAgentCharacters(agentName: string): string {
 }
 
 export function stripAgentListSortPrefix(agentName: string): string {
-  return stripInvisibleAgentCharacters(agentName)
+  return stripInvisibleAgentCharacters(agentName).replace(/^\s+/, "")
 }
 
 export function getAgentRuntimeName(configKey: string): string {

src/shared/agent-runtime-name-sort.test.ts

@@ -0,0 +1,152 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, it, test } from "bun:test"
+
+import {
+  AGENT_DISPLAY_NAMES,
+  getAgentRuntimeName,
+  normalizeAgentForPromptKey,
+} from "./agent-display-names"
+
+// OpenCode Agent.list() sorts via remeda sortBy: default_agent desc, then name asc localeCompare.
+// Reference: ../opencode/packages/opencode/src/agent/agent.ts:284-293.
+// Earlier ZWSP prefixes silently failed: Unicode collation treats zero-width chars as ignorable.
+function simulateOpencodeSort(agentNames: string[], defaultName: string): string[] {
+  return [...agentNames].sort((a, b) => {
+    const aIsDefault = a === defaultName ? 1 : 0
+    const bIsDefault = b === defaultName ? 1 : 0
+    if (aIsDefault !== bIsDefault) return bIsDefault - aIsDefault
+    return a.localeCompare(b)
+  })
+}
+
+describe("OpenCode Agent.list() sort with runtime-name prefixes", () => {
+  describe("#given the four core agents and a mix of non-core agents", () => {
+    test("#when sorted using opencode-style sortBy #then core agents come first in canonical order", () => {
+      const sisyphus = getAgentRuntimeName("sisyphus")
+      const hephaestus = getAgentRuntimeName("hephaestus")
+      const prometheus = getAgentRuntimeName("prometheus")
+      const atlas = getAgentRuntimeName("atlas")
+
+      const allAgents = [
+        sisyphus,
+        hephaestus,
+        prometheus,
+        atlas,
+        "athena",
+        "explore",
+        "metis",
+        "oracle",
+      ]
+
+      const sorted = simulateOpencodeSort(allAgents, sisyphus)
+      const orderedConfigKeys = sorted.map((name) => normalizeAgentForPromptKey(name))
+
+      expect(orderedConfigKeys).toEqual([
+        "sisyphus",
+        "hephaestus",
+        "prometheus",
+        "atlas",
+        "athena",
+        "explore",
+        "metis",
+        "oracle",
+      ])
+    })
+
+    test("#when default_agent is unset #then canonical core order still holds via prefix alone", () => {
+      const sisyphus = getAgentRuntimeName("sisyphus")
+      const hephaestus = getAgentRuntimeName("hephaestus")
+      const prometheus = getAgentRuntimeName("prometheus")
+      const atlas = getAgentRuntimeName("atlas")
+
+      const allAgents = [hephaestus, prometheus, atlas, sisyphus, "athena", "oracle"]
+
+      const sorted = simulateOpencodeSort(allAgents, "no-such-default-agent")
+      const orderedConfigKeys = sorted.map((name) => normalizeAgentForPromptKey(name))
+
+      expect(orderedConfigKeys.slice(0, 4)).toEqual([
+        "sisyphus",
+        "hephaestus",
+        "prometheus",
+        "atlas",
+      ])
+    })
+  })
+
+  describe("#given input array in random order", () => {
+    test("#when sorted with opencode comparator #then result is always canonical", () => {
+      const sisyphus = getAgentRuntimeName("sisyphus")
+      const hephaestus = getAgentRuntimeName("hephaestus")
+      const prometheus = getAgentRuntimeName("prometheus")
+      const atlas = getAgentRuntimeName("atlas")
+      const nonCore = ["athena", "explore", "librarian", "metis", "oracle"]
+      const allAgents = [...nonCore, atlas, prometheus, hephaestus, sisyphus]
+
+      for (let attempt = 0; attempt < 25; attempt += 1) {
+        const shuffled = [...allAgents]
+        for (let i = shuffled.length - 1; i > 0; i -= 1) {
+          const j = Math.floor(Math.random() * (i + 1))
+          ;[shuffled[i], shuffled[j]] = [shuffled[j], shuffled[i]]
+        }
+        const sorted = simulateOpencodeSort(shuffled, sisyphus)
+        const orderedConfigKeys = sorted.map((name) => normalizeAgentForPromptKey(name))
+
+        expect(orderedConfigKeys).toEqual([
+          "sisyphus",
+          "hephaestus",
+          "prometheus",
+          "atlas",
+          "athena",
+          "explore",
+          "librarian",
+          "metis",
+          "oracle",
+        ])
+      }
+    })
+  })
+
+  describe("#given runtime names containing only core agents", () => {
+    test("#when sorted #then sisyphus, hephaestus, prometheus, atlas in that order", () => {
+      const sisyphus = getAgentRuntimeName("sisyphus")
+      const hephaestus = getAgentRuntimeName("hephaestus")
+      const prometheus = getAgentRuntimeName("prometheus")
+      const atlas = getAgentRuntimeName("atlas")
+
+      const sorted = simulateOpencodeSort([atlas, prometheus, hephaestus, sisyphus], sisyphus)
+      const orderedConfigKeys = sorted.map((name) => normalizeAgentForPromptKey(name))
+
+      expect(orderedConfigKeys).toEqual([
+        "sisyphus",
+        "hephaestus",
+        "prometheus",
+        "atlas",
+      ])
+    })
+  })
+
+  describe("#given the prefix is meant to render in OpenCode TUI", () => {
+    it("uses ASCII whitespace so terminals render the prefix without character corruption", () => {
+      const runtimeNames = Object.keys(AGENT_DISPLAY_NAMES).map(getAgentRuntimeName)
+      const invisibleCharsRegex = /[\u200B\u200C\u200D\uFEFF]/
+
+      for (const name of runtimeNames) {
+        expect(invisibleCharsRegex.test(name)).toBe(false)
+      }
+    })
+
+    it("only adds leading whitespace, never trailing or interior whitespace beyond the display name", () => {
+      const sisyphus = getAgentRuntimeName("sisyphus")
+      const hephaestus = getAgentRuntimeName("hephaestus")
+      const prometheus = getAgentRuntimeName("prometheus")
+      const atlas = getAgentRuntimeName("atlas")
+
+      for (const name of [sisyphus, hephaestus, prometheus, atlas]) {
+        const trimmed = name.trimStart()
+        expect(name.length).toBeGreaterThanOrEqual(trimmed.length)
+        expect(trimmed.endsWith(" ")).toBe(false)
+      }
+    })
+  })
+})