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/agent-dispatcher.ts

@@ -0,0 +1,90 @@
+// Agent Dispatcher — connects sidecar bridge events to agent store
+// Single listener that routes sidecar messages to the correct agent session
+
+import { onSidecarMessage, type SidecarMessage } from './adapters/agent-bridge';
+import { adaptSDKMessage } from './adapters/sdk-messages';
+import type { InitContent, CostContent } from './adapters/sdk-messages';
+import {
+  updateAgentStatus,
+  setAgentSdkSessionId,
+  setAgentModel,
+  appendAgentMessages,
+  updateAgentCost,
+} from './stores/agents';
+
+let unlistenFn: (() => void) | null = null;
+
+export async function startAgentDispatcher(): Promise<void> {
+  if (unlistenFn) return;
+
+  unlistenFn = await onSidecarMessage((msg: SidecarMessage) => {
+    const sessionId = msg.sessionId;
+    if (!sessionId) return;
+
+    switch (msg.type) {
+      case 'agent_started':
+        updateAgentStatus(sessionId, 'running');
+        break;
+
+      case 'agent_event':
+        handleAgentEvent(sessionId, msg.event!);
+        break;
+
+      case 'agent_stopped':
+        updateAgentStatus(sessionId, 'done');
+        break;
+
+      case 'agent_error':
+        updateAgentStatus(sessionId, 'error', msg.message);
+        break;
+
+      case 'agent_log':
+        // Debug logging — could route to a log panel later
+        break;
+    }
+  });
+}
+
+function handleAgentEvent(sessionId: string, event: Record<string, unknown>): void {
+  const messages = adaptSDKMessage(event);
+
+  for (const msg of messages) {
+    switch (msg.type) {
+      case 'init': {
+        const init = msg.content as InitContent;
+        setAgentSdkSessionId(sessionId, init.sessionId);
+        setAgentModel(sessionId, init.model);
+        break;
+      }
+
+      case 'cost': {
+        const cost = msg.content as CostContent;
+        updateAgentCost(sessionId, {
+          costUsd: cost.totalCostUsd,
+          inputTokens: cost.inputTokens,
+          outputTokens: cost.outputTokens,
+          numTurns: cost.numTurns,
+          durationMs: cost.durationMs,
+        });
+        if (cost.isError) {
+          updateAgentStatus(sessionId, 'error', cost.errors?.join('; '));
+        } else {
+          updateAgentStatus(sessionId, 'done');
+        }
+        break;
+      }
+    }
+  }
+
+  // Append all messages to the session history
+  if (messages.length > 0) {
+    appendAgentMessages(sessionId, messages);
+  }
+}
+
+export function stopAgentDispatcher(): void {
+  if (unlistenFn) {
+    unlistenFn();
+    unlistenFn = null;
+  }
+}