fix(app): restore per-session agent selection when switching between sessions

[PriyeshPandey2000]

Summary

• Agent selection lived in one global slot (local.prefs.selectedAgent) shared by every session, so picking an agent in one conversation overwrote it for all of them
• Adds session-agent-store.ts, a localStorage store keyed by session ID, mirroring the existing session-model-store.ts
• session-route.tsx reads/writes the active session's agent; the new-task composer still uses the global pref

Why

• Returning to a conversation silently changed its agent, so the next message ran under the wrong one
• Per-session model choice already works this way; agent selection was the outlier

Issue

• Closes [Bug]: Chosen agent per session keep being overwritten by the latest chat #3461

Scope

• session-agent-store.ts (new): per-session agent map under openwork.sessionAgents.v1, capped at 200 entries, tolerant of missing or malformed storage
• session-route.tsx: selectedAgent derives from the active session; setSelectedAgent writes per-session, or to the global pref when no session is selected

Out of scope

• local.prefs.selectedAgent stays as the new-task composer's source of truth, preserving the reload persistence from #2101
• New sessions start at the default agent rather than inheriting the composer's choice — session-model-store.ts behaves the same way today, so this keeps the two consistent rather than changing one in a bugfix
• No server, schema, or sync changes — local UI state only

Testing

Ran

• pnpm typecheck
• bun test --isolate tests/ (apps/app unit suite)
• pnpm test:session-switch
• pnpm test:sessions
• pnpm test:health

Result

• pass/fail: pass
• if fail, exact files/errors: tests/message-list-loading.test.tsx has 2 failures (Invalid hook call / TypeError: null is not an object (evaluating 'resolveDispatcher().useState') in renderToStringImpl). These are pre-existing on dev — I re-ran that file with this branch's two files reverted to their dev state and got the identical 2 failures, so they are unrelated to this change. Everything else is green: 600 pass in the unit suite, 7/7 steps in test:session-switch, ok: true for test:sessions and test:health.

CI status

• pass: typecheck, session-switch, sessions, health
• code-related failures: none from this change
• external/env/auth blockers: none

Manual verification

1. Set the build agent on session A and plan on session B
2. Switch back to session A — the picker shows build (before this change it showed plan)
3. Reload the app and switch between both sessions — each still restores its own agent
4. With no session selected, pick an agent in the new-task composer and reload — the choice is restored
5. Clear a session's agent back to default — the entry is removed and the session falls back to the default agent

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Project	Deployment	Actions	Updated (UTC)
openwork-landing	Ready	Preview, v0	Aug 3, 2026 6:17pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
openwork-diagnostics	Skipped		Aug 3, 2026 6:17pm

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