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/components/panes/ChatPane.tsx

@@ -18,10 +18,13 @@ interface Props {
   sessionId: string;
   chatId: string;
   projectId: string;
+  // Batch 9: optional, threaded down to ChatInput's agent picker.
+  agentId?: string | null;
+  onAgentChange?: (agentId: string | null) => void | Promise<void>;
   sessionChats?: import('@/api/types').Chat[];
 }
 
-export function ChatPane({ sessionId, chatId, projectId, sessionChats }: Props) {
+export function ChatPane({ sessionId, chatId, projectId, agentId, onAgentChange, sessionChats }: Props) {
   const stream = useSessionStream(sessionId);
   const lastErrorRef = useRef<string | null>(null);
   const [queue, setQueue] = useState<string[]>([]);
@@ -167,7 +170,14 @@ export function ChatPane({ sessionId, chatId, projectId, sessionChats }: Props)
 
       <div className="relative">
         <ChatContextPopover stats={contextStats} />
-        <ChatInput disabled={false} projectId={projectId} onSend={handleSend} onForceSend={streaming ? handleForceSend : undefined} />
+        <ChatInput
+          disabled={false}
+          projectId={projectId}
+          agentId={agentId}
+          onAgentChange={onAgentChange}
+          onSend={handleSend}
+          onForceSend={streaming ? handleForceSend : undefined}
+        />
       </div>
     </div>
   );