feat(agents): Tier 2 — AGENTS.md + per-session picker

Six builtin defaults (Code Reviewer, Debugger, Refactorer, Architect,
Security Auditor, Prompt Builder) with no model field so session.model
wins. Project root AGENTS.md parsed on demand with mtime cache; when
present, only its agents are shown. sessions.agent_id resolves per turn
into effective system prompt, temperature, and a tool whitelist applied
in inference. AgentPicker mounts in the ChatInput toolbar; SettingsDrawer
agent surface deferred to Batch 7.

apps/server/src/index.ts

@@ -14,6 +14,7 @@ import { registerChatRoutes } from './routes/chats.js';
 import { registerSidebarRoutes } from './routes/sidebar.js';
 import { registerWebSocket } from './routes/ws.js';
 import { registerModelRoutes } from './routes/models.js';
+import { registerAgentRoutes } from './routes/agents.js';
 import { createInferenceRunner } from './services/inference.js';
 import { createBroker } from './services/broker.js';
 
@@ -57,6 +58,7 @@ async function main() {
   registerSessionRoutes(app, sql, config, broker);
   registerSettingsRoutes(app, sql);
   registerModelRoutes(app, config);
+  registerAgentRoutes(app, sql);
   registerSidebarRoutes(app, sql);
   registerChatRoutes(app, sql, broker);