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/web/src/pages/Session.tsx

@@ -193,7 +193,15 @@ export function Session() {
       </header>
 
       {id && session && (
-        <Workspace sessionId={id} projectId={session.project_id} />
+        <Workspace
+          sessionId={id}
+          projectId={session.project_id}
+          agentId={session.agent_id}
+          onAgentChange={async (agent_id) => {
+            const updated = await api.sessions.update(session.id, { agent_id });
+            setSession(updated);
+          }}
+        />
       )}
     </div>
   );