fix(ui): scope primary agent selector to selectable agents (#528)

## Summary

- rescope the UI portion of #385 to the remaining main-session selector
issue from #384
- keep main-session primary selectors limited to selectable primary
agents
- preserve child/subagent behavior by keeping the current hidden agent
available for steering
- add focused tests for selector eligibility behavior

This intentionally excludes the OMO/config-directory merge and plugin
retry scope from #385, since that part was determined to belong outside
CodeNomad. Credit to @jollyxenon for the original #385 investigation and
UI direction.

## Validation

- ode --test packages/ui/src/types/session.test.ts ✅
- pm run typecheck --workspace @codenomad/ui ⚠️ fails on existing
SDK/type drift outside this diff

Fixes #384
Based on #385

packages/ui/src/components/agent-selector.tsx

@@ -2,7 +2,7 @@ import { Select } from "@kobalte/core/select"
 import { Show, createEffect, createMemo } from "solid-js"
 import { agents, fetchAgents, sessions } from "../stores/sessions"
 import { ChevronDown } from "lucide-solid"
-import { isSelectablePrimaryAgent, type Agent } from "../types/session"
+import { getSelectableAgentsForSession, type Agent } from "../types/session"
 import { useI18n } from "../lib/i18n"
 import { getLogger } from "../lib/logger"
 const log = getLogger("session")
@@ -29,12 +29,7 @@ export default function AgentSelector(props: AgentSelectorProps) {
   })
 
   const availableAgents = createMemo(() => {
-    const allAgents = instanceAgents()
-    if (isChildSession()) {
-      return allAgents.filter((agent) => !agent.hidden)
-    }
-
-    return allAgents.filter(isSelectablePrimaryAgent)
+    return getSelectableAgentsForSession(instanceAgents(), props.currentAgent, isChildSession())
   })
 
   createEffect(() => {

packages/ui/src/types/session.test.ts

@@ -0,0 +1,45 @@
+import assert from "node:assert/strict"
+import { describe, it } from "node:test"
+
+import { getSelectableAgentsForSession, isSelectablePrimaryAgent, type Agent } from "./session.ts"
+
+const visiblePrimary: Agent = { name: "plan", description: "", mode: "primary" }
+const visibleSubagent: Agent = { name: "review", description: "", mode: "subagent" }
+const hiddenPrimary: Agent = { name: "build", description: "", mode: "primary", hidden: true }
+const hiddenSubagent: Agent = { name: "debug", description: "", mode: "subagent", hidden: true }
+
+describe("agent selectability", () => {
+  it("matches primary-session selector rules", () => {
+    assert.equal(isSelectablePrimaryAgent(visiblePrimary), true)
+    assert.equal(isSelectablePrimaryAgent(visibleSubagent), false)
+    assert.equal(isSelectablePrimaryAgent(hiddenPrimary), false)
+    assert.equal(isSelectablePrimaryAgent(hiddenSubagent), false)
+  })
+
+  it("excludes hidden and subagent entries from main-session selectors", () => {
+    const agents = [hiddenPrimary, visibleSubagent, visiblePrimary]
+
+    assert.deepEqual(
+      getSelectableAgentsForSession(agents, "build", false).map((agent) => agent.name),
+      ["plan"],
+    )
+  })
+
+  it("preserves a child session's current hidden agent for steering", () => {
+    const agents = [hiddenPrimary, visibleSubagent, visiblePrimary]
+
+    assert.deepEqual(
+      getSelectableAgentsForSession(agents, "build", true).map((agent) => agent.name),
+      ["review", "plan", "build"],
+    )
+  })
+
+  it("does not add unrelated hidden agents to child-session selectors", () => {
+    const agents = [hiddenPrimary, visibleSubagent, visiblePrimary]
+
+    assert.deepEqual(
+      getSelectableAgentsForSession(agents, "review", true).map((agent) => agent.name),
+      ["review", "plan"],
+    )
+  })
+})

packages/ui/src/types/session.ts

@@ -120,6 +120,23 @@ export function isSelectablePrimaryAgent(agent: Agent): boolean {
   return !agent.hidden && agent.mode !== "subagent"
 }
 
+export function getSelectableAgentsForSession(
+  agentList: Agent[],
+  currentAgentName: string,
+  isChildSession: boolean,
+): Agent[] {
+  if (!isChildSession) {
+    return agentList.filter(isSelectablePrimaryAgent)
+  }
+
+  const visibleAgents = agentList.filter((agent) => !agent.hidden)
+  const currentHiddenAgent = agentList.find((agent) => agent.hidden && agent.name === currentAgentName)
+
+  return currentHiddenAgent && !visibleAgents.some((agent) => agent.name === currentHiddenAgent.name)
+    ? [...visibleAgents, currentHiddenAgent]
+    : visibleAgents
+}
+
 // Our client-specific Provider interface (simplified version of SDK Provider)
 export interface Provider {
   id: string