fix: respect default_agent in sort shim ordering (#3900)

The sort shim always placed core agents (sisyphus, hephaestus, prometheus,
atlas) before any custom agent, ignoring the user's configured default_agent
in opencode.json. This caused the TUI to always show Sisyphus as the selected
agent on startup regardless of default_agent.

Add setDefaultAgentForSort() which inserts the configured default_agent at
rank 0 in the sort shim's rank map. Called from applyAgentConfig after
resolving the effective default_agent value.

Fixes #3900

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

@@ -8,6 +8,7 @@ import {
   normalizeAgentForPromptKey,
 } from "../shared/agent-display-names";
 import { AGENT_NAME_MAP } from "../shared/migration";
+import { setDefaultAgentForSort } from "../shared/agent-sort-shim";
 import { registerAgentName } from "../features/claude-code-session-state";
 import {
   discoverConfigSourceSkills,
@@ -399,6 +400,11 @@ export async function applyAgentConfig(params: {
     );
   }
 
+  const resolvedDefault = (params.config as { default_agent?: string }).default_agent;
+  if (resolvedDefault) {
+    setDefaultAgentForSort(resolvedDefault);
+  }
+
   const agentResult = params.config.agent as Record<string, unknown>;
   for (const name of Object.keys(agentResult)) {
     registerAgentName(name);

src/shared/agent-sort-shim.test.ts

@@ -2,7 +2,7 @@
 
 import { afterEach, beforeAll, describe, expect, test } from "bun:test"
 
-import { installAgentSortShim, setAgentSortOrder } from "./agent-sort-shim"
+import { installAgentSortShim, setAgentSortOrder, setDefaultAgentForSort } from "./agent-sort-shim"
 import { AGENT_DISPLAY_NAMES } from "./agent-display-names"
 
 type AgentListItem = {
@@ -224,4 +224,45 @@ describe("agent-sort-shim", () => {
       })
     })
   })
+
+  describe("#given a custom default_agent configured via setDefaultAgentForSort", () => {
+    describe("#when toSorted is called on core agents mixed with the custom default agent", () => {
+      test("#then the custom default agent sorts first, followed by core agents in canonical order", () => {
+        // given
+        setAgentSortOrder(undefined)
+        setDefaultAgentForSort("crystal")
+        const sisyphus = { name: "Sisyphus - Ultraworker" }
+        const hephaestus = { name: "Hephaestus - Deep Agent" }
+        const prometheus = { name: "Prometheus - Plan Builder" }
+        const atlas = { name: "Atlas - Plan Executor" }
+        const crystal = { name: "crystal" }
+        const input = [atlas, crystal, prometheus, hephaestus, sisyphus]
+
+        // when
+        const result = input.toSorted((a, b) => a.name.localeCompare(b.name))
+
+        // then
+        expect(result).toEqual([crystal, sisyphus, hephaestus, prometheus, atlas])
+      })
+    })
+
+    describe("#when setDefaultAgentForSort is called with a core agent name", () => {
+      test("#then that core agent sorts first, others follow in remaining canonical order", () => {
+        // given
+        setAgentSortOrder(undefined)
+        setDefaultAgentForSort("Hephaestus - Deep Agent")
+        const sisyphus = { name: "Sisyphus - Ultraworker" }
+        const hephaestus = { name: "Hephaestus - Deep Agent" }
+        const prometheus = { name: "Prometheus - Plan Builder" }
+        const atlas = { name: "Atlas - Plan Executor" }
+        const input = [atlas, prometheus, hephaestus, sisyphus]
+
+        // when
+        const result = input.toSorted((a, b) => a.name.localeCompare(b.name))
+
+        // then
+        expect(result).toEqual([hephaestus, sisyphus, prometheus, atlas])
+      })
+    })
+  })
 })

src/shared/agent-sort-shim.ts

@@ -82,6 +82,17 @@ export function setAgentSortOrder(agentOrder: readonly string[] | undefined): vo
   agentRank = createAgentRank(agentOrder)
 }
 
+export function setDefaultAgentForSort(agentName: string | undefined): void {
+  if (!agentName) return
+  if (agentRank.get(agentName) === 0) return
+  const updated = new Map<string, number>()
+  updated.set(agentName, 0)
+  for (const [key, rank] of agentRank) {
+    if (key !== agentName) updated.set(key, rank + 1)
+  }
+  agentRank = updated
+}
+
 export function installAgentSortShim(): void {
   if (installed) return