feat(v2): add agent pane with SDK message adapter and dispatcher

Implement full agent session frontend: SDK message adapter parsing
stream-json into 9 typed message types, agent bridge for Tauri IPC,
dispatcher routing sidecar events to store, agent session store with
cost tracking, and AgentPane component with prompt input, message
rendering (text, thinking, tool calls, results, cost), and stop
button. Add Ctrl+Shift+N shortcut and sidebar agent button.

v2/src/lib/adapters/agent-bridge.ts

@@ -0,0 +1,49 @@
+// Agent Bridge — Tauri IPC adapter for sidecar communication
+// Mirrors pty-bridge.ts pattern: invoke for commands, listen for events
+
+import { invoke } from '@tauri-apps/api/core';
+import { listen, type UnlistenFn } from '@tauri-apps/api/event';
+
+export interface AgentQueryOptions {
+  session_id: string;
+  prompt: string;
+  cwd?: string;
+  max_turns?: number;
+  max_budget_usd?: number;
+  resume_session_id?: string;
+}
+
+export async function queryAgent(options: AgentQueryOptions): Promise<void> {
+  return invoke('agent_query', { options });
+}
+
+export async function stopAgent(sessionId: string): Promise<void> {
+  return invoke('agent_stop', { sessionId });
+}
+
+export async function isAgentReady(): Promise<boolean> {
+  return invoke<boolean>('agent_ready');
+}
+
+export interface SidecarMessage {
+  type: string;
+  sessionId?: string;
+  event?: Record<string, unknown>;
+  message?: string;
+  exitCode?: number | null;
+  signal?: string | null;
+}
+
+export async function onSidecarMessage(
+  callback: (msg: SidecarMessage) => void,
+): Promise<UnlistenFn> {
+  return listen<SidecarMessage>('sidecar-message', (event) => {
+    callback(event.payload as SidecarMessage);
+  });
+}
+
+export async function onSidecarExited(callback: () => void): Promise<UnlistenFn> {
+  return listen('sidecar-exited', () => {
+    callback();
+  });
+}