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/plugin-handlers/AGENTS.md

@@ -8,28 +8,45 @@ The canonical agent order is **sisyphus → hephaestus → prometheus → atlas*
 
 This order is enforced via two mechanisms working together:
 1. `CANONICAL_CORE_AGENT_ORDER` in `agent-priority-order.ts` controls object key insertion order
-2. `agent-key-remapper.ts` injects ZWSP-prefixed runtime names into the `name` field for OpenCode's `localeCompare` sort
+2. `agent-key-remapper.ts` injects leading-space-prefixed runtime names into the `name` field for OpenCode's `localeCompare` sort
 
 ### Why Two Mechanisms
 
-OpenCode's `Agent.list()` sorts agents by `name` field via `localeCompare`. Object key order alone is not enough. The `name` field carries ZWSP prefixes (1-4 chars) so core agents sort before alphabetically-named agents.
+OpenCode's `Agent.list()` sorts agents by `name` field via `localeCompare`. Object key order alone is not enough. The `name` field carries leading ASCII spaces (4-3-2-1 descending) so core agents sort before alphabetically-named agents.
 
-ZWSP is intentionally used in the `name` field only. It MUST NOT appear in:
+The prefix lengths are intentionally **descending** (sisyphus=4, hephaestus=3, prometheus=2, atlas=1) because `localeCompare` puts strings with more leading whitespace before strings with fewer. Reference: see `agent-runtime-name-sort.test.ts` for empirical verification.
+
+### Why ASCII Spaces, Not ZWSP
+
+Earlier versions used ZWSP (`\u200B`) prefixes hoping they would be invisible to users. They silently failed: Unicode collation algorithms treat zero-width characters as ignorable at the primary level, so ZWSP-prefixed names sorted as if the prefix did not exist. The result was alphabetical order interleaving core and non-core agents.
+
+ASCII space (`\u0020`) is the only character that:
+- Sorts before alphabetic characters reliably under all locales
+- Renders correctly in every terminal (no glyph substitution)
+- Is valid in HTTP header values (RFC 7230) when placed in the `name` field
+
+The leading-space prefix MUST NOT appear in:
 - Object keys (used as HTTP header values, causes RFC 7230 violations)
 - Display names returned by `getAgentDisplayName()`
 - Config keys
 
+### Backward Compatibility
+
+`stripAgentListSortPrefix()` strips both the new leading-space prefix AND legacy ZWSP/zero-width characters. Existing sessions and configs from the ZWSP era continue to resolve correctly.
+
 ### History
 
-Agent ordering has caused 15+ commits, 8+ PRs, and multiple reverts due to:
+Agent ordering caused 15+ commits, 8+ PRs, and multiple reverts due to:
 1. Early ZWSP attempts that leaked into HTTP headers via object keys
 2. Object.entries() iteration order depending on merge sequence
 3. Multiple code paths assembling agents differently
+4. The ZWSP prefix being silently broken in `localeCompare` sort (resolved in this commit by switching to leading ASCII spaces)
 
 ### Forbidden Patterns
 
 DO NOT introduce:
-- ZWSP in object keys or display names (only allowed in `name` field via `getAgentRuntimeName()`)
+- ZWSP in any field (broken in `localeCompare`, replaced by leading ASCII spaces)
+- Leading whitespace in object keys or display names (allowed only in `name` field via `getAgentRuntimeName()`)
 - Runtime sort shims or comparators
 - Alternative ordering constants
 - Object.entries() order dependencies

src/plugin-handlers/agent-config-handler.ts

@@ -2,7 +2,7 @@ import { createBuiltinAgents } from "../agents";
 import { createSisyphusJuniorAgentWithOverrides } from "../agents/sisyphus-junior";
 import type { OhMyOpenCodeConfig } from "../config";
 import { isTaskSystemEnabled, log, migrateAgentConfig } from "../shared";
-import { getAgentRuntimeName } from "../shared/agent-display-names";
+import { AGENT_DISPLAY_NAMES, getAgentConfigKey, getAgentRuntimeName } from "../shared/agent-display-names";
 import { AGENT_NAME_MAP } from "../shared/migration";
 import { registerAgentName } from "../features/claude-code-session-state";
 import {
@@ -189,8 +189,11 @@ export async function applyAgentConfig(params: {
 
   if (isSisyphusEnabled && builtinAgents.sisyphus) {
     if (configuredDefaultAgent) {
-      (params.config as { default_agent?: string }).default_agent =
-        getAgentRuntimeName(configuredDefaultAgent);
+      const configKey = getAgentConfigKey(configuredDefaultAgent);
+      const isKnownBuiltin = configKey in AGENT_DISPLAY_NAMES;
+      (params.config as { default_agent?: string }).default_agent = isKnownBuiltin
+        ? getAgentRuntimeName(configKey)
+        : configuredDefaultAgent;
     } else {
       (params.config as { default_agent?: string }).default_agent =
         getAgentRuntimeName("sisyphus");