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.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/server/src/routes/agents.ts

@@ -0,0 +1,20 @@
+import type { FastifyInstance } from 'fastify';
+import type { Sql } from '../db.js';
+import { getAgentsForProject } from '../services/agents.js';
+
+export function registerAgentRoutes(app: FastifyInstance, sql: Sql): void {
+  app.get<{ Params: { id: string } }>(
+    '/api/projects/:id/agents',
+    async (req, reply) => {
+      const rows = await sql<{ path: string }[]>`
+        SELECT path FROM projects WHERE id = ${req.params.id}
+      `;
+      if (rows.length === 0) {
+        reply.code(404);
+        return { error: 'project not found' };
+      }
+      // getAgentsForProject handles AGENTS.md presence/parse/cache; never throws.
+      return await getAgentsForProject(rows[0]!.path);
+    }
+  );
+}