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/ChatInput.tsx

@@ -15,6 +15,7 @@ import { AttachmentChip } from '@/components/AttachmentChip';
 import { AttachmentPreviewModal } from '@/components/AttachmentPreviewModal';
 import { FileMentionPopover } from '@/components/FileMentionPopover';
 import { DropOverlay } from '@/components/DropOverlay';
+import { AgentPicker } from '@/components/AgentPicker';
 import { api } from '@/api/client';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { useViewport } from '@/hooks/useViewport';
@@ -24,11 +25,15 @@ const MAX_ATTACHMENTS = 10;
 interface Props {
   disabled?: boolean;
   projectId: string;
+  // Batch 9: optional so callers that pre-date the agent picker still compile.
+  // When omitted, the toolbar row is hidden entirely.
+  agentId?: string | null;
+  onAgentChange?: (agentId: string | null) => void | Promise<void>;
   onSend: (content: string) => void | Promise<void>;
   onForceSend?: (content: string) => void | Promise<void>;
 }
 
-export function ChatInput({ disabled, projectId, onSend, onForceSend }: Props) {
+export function ChatInput({ disabled, projectId, agentId, onAgentChange, onSend, onForceSend }: Props) {
   const { isMobile } = useViewport();
   const [value, setValue] = useState('');
   const [busy, setBusy] = useState(false);
@@ -420,6 +425,18 @@ export function ChatInput({ disabled, projectId, onSend, onForceSend }: Props) {
           ))}
         </div>
       )}
+      {/* Batch 9 toolbar — agent picker. Sits above the input row so it
+          doesn't compete with the send button for vertical alignment.
+          When Batch 7 lands, ModelPicker and the + button join this row. */}
+      {onAgentChange && (
+        <div className="px-4 pt-2 flex items-center gap-1.5">
+          <AgentPicker
+            projectId={projectId}
+            value={agentId ?? null}
+            onChange={onAgentChange}
+          />
+        </div>
+      )}
       <div className="px-4 py-3 flex items-end gap-2">
         <Textarea
           ref={textareaRef}