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/Workspace.tsx

@@ -20,9 +20,12 @@ import { cn } from '@/lib/utils';
 interface Props {
   sessionId: string;
   projectId: string;
+  // Batch 9: threaded down to ChatPane → ChatInput → AgentPicker.
+  agentId?: string | null;
+  onAgentChange?: (agentId: string | null) => void | Promise<void>;
 }
 
-export function Workspace({ sessionId, projectId }: Props) {
+export function Workspace({ sessionId, projectId, agentId, onAgentChange }: Props) {
   const {
     panes,
     activePaneIdx,
@@ -219,7 +222,14 @@ export function Workspace({ sessionId, projectId }: Props) {
 
             <div className="flex-1 min-h-0 overflow-hidden">
               {pane.kind === 'chat' && pane.chatId ? (
-                <ChatPane sessionId={sessionId} chatId={pane.chatId} projectId={projectId} sessionChats={chats} />
+                <ChatPane
+                  sessionId={sessionId}
+                  chatId={pane.chatId}
+                  projectId={projectId}
+                  agentId={agentId}
+                  onAgentChange={onAgentChange}
+                  sessionChats={chats}
+                />
               ) : (
                 <SessionLandingPage
                   sessionId={sessionId}