feat(booterm): add PTY session registry + listing endpoint

In-memory SessionMeta registry tracks active terminal sessions with
paneId, sessionId, projectPath, title, createdAt, lastActivityAt.
GET /api/term/sessions returns all active sessions as JSON array.
Registry is updated on WS attach and cleaned up on disconnect.

apps/booterm/src/pty/registry.ts

@@ -0,0 +1,44 @@
+export interface SessionMeta {
+  paneId: string;
+  sessionId: string;
+  projectPath: string;
+  title?: string;
+  createdAt: Date;
+  lastActivityAt: Date;
+}
+
+const sessions = new Map<string, SessionMeta>();
+
+export function register(
+  sessionId: string,
+  paneId: string,
+  projectPath: string,
+  title?: string,
+): void {
+  const now = new Date();
+  const existing = sessions.get(paneId);
+  if (existing) {
+    existing.lastActivityAt = now;
+    return;
+  }
+  sessions.set(paneId, {
+    paneId,
+    sessionId,
+    projectPath,
+    title,
+    createdAt: now,
+    lastActivityAt: now,
+  });
+}
+
+export function unregister(paneId: string): void {
+  sessions.delete(paneId);
+}
+
+export function list(): SessionMeta[] {
+  return Array.from(sessions.values());
+}
+
+export function get(paneId: string): SessionMeta | undefined {
+  return sessions.get(paneId);
+}