feat(server): add pty-session-cookie module for the Terminal tab

Mirrors `sse-session-cookie.ts` exactly. Mints short-lived 30-min HttpOnly
cookies for authenticating the Terminal-tab WebSocket upgrade against
the terminal-agent. Same TTL, same opportunistic-pruning shape, same
"scoped tokens never valid as root" invariant. Two registries instead of
one because the cookie names are different (`gstack_sse` vs `gstack_pty`)
and the token spaces must not overlap.

No callers yet — wired up in the next commit.

browse/src/pty-session-cookie.ts

@@ -0,0 +1,122 @@
+/**
+ * Session cookie registry for the Terminal sidebar tab's PTY WebSocket.
+ *
+ * Why this exists: WebSocket clients in browsers cannot send Authorization
+ * headers on the upgrade request. The terminal-agent's /ws upgrade therefore
+ * authenticates via cookie. We never put the PTY token in /health (codex
+ * outside-voice finding #2: /health already leaks AUTH_TOKEN to any
+ * localhost caller in headed mode; reusing that path for shell access would
+ * widen an existing bug). Instead, the extension does an authenticated
+ * POST /pty-session with the bootstrap AUTH_TOKEN; the server mints a
+ * short-lived cookie scoped to this terminal session and pushes it to the
+ * agent via loopback. The browser then carries the cookie automatically on
+ * the WS upgrade.
+ *
+ * Design mirrors `sse-session-cookie.ts` deliberately. Same TTL, same
+ * scoped-token-must-not-be-valid-as-root invariant, same opportunistic
+ * pruning. Two registries instead of one because the cookie names are
+ * different (`gstack_sse` vs `gstack_pty`) and the token spaces must not
+ * overlap — an SSE-read cookie must never grant PTY access, and vice versa.
+ */
+import * as crypto from 'crypto';
+
+interface Session {
+  createdAt: number;
+  expiresAt: number;
+}
+
+const TTL_MS = 30 * 60 * 1000; // 30 minutes — matches SSE cookie
+const MAX_SESSIONS = 10_000;
+const sessions = new Map<string, Session>();
+
+export const PTY_COOKIE_NAME = 'gstack_pty';
+
+/** Mint a fresh PTY session token. */
+export function mintPtySessionToken(): { token: string; expiresAt: number } {
+  const token = crypto.randomBytes(32).toString('base64url');
+  const now = Date.now();
+  const expiresAt = now + TTL_MS;
+  sessions.set(token, { createdAt: now, expiresAt });
+  pruneExpired(now);
+  return { token, expiresAt };
+}
+
+/**
+ * Validate a token. Returns true only if the token exists AND is not expired.
+ * Lazily removes expired entries; opportunistically prunes a few more on
+ * every call so the registry stays bounded under reconnect pressure.
+ */
+export function validatePtySessionToken(token: string | null | undefined): boolean {
+  if (!token) return false;
+  const s = sessions.get(token);
+  if (!s) {
+    pruneExpired(Date.now());
+    return false;
+  }
+  if (Date.now() > s.expiresAt) {
+    sessions.delete(token);
+    pruneExpired(Date.now());
+    return false;
+  }
+  return true;
+}
+
+/**
+ * Drop a session token (called on WS close so a leaked cookie can't be
+ * replayed against a new PTY).
+ */
+export function revokePtySessionToken(token: string | null | undefined): void {
+  if (!token) return;
+  sessions.delete(token);
+}
+
+/** Parse the PTY session token from a Cookie header. */
+export function extractPtyCookie(req: Request): string | null {
+  const cookieHeader = req.headers.get('cookie');
+  if (!cookieHeader) return null;
+  for (const part of cookieHeader.split(';')) {
+    const [name, ...valueParts] = part.trim().split('=');
+    if (name === PTY_COOKIE_NAME) {
+      return valueParts.join('=') || null;
+    }
+  }
+  return null;
+}
+
+/**
+ * Build the Set-Cookie header value for the PTY session cookie.
+ * - HttpOnly: not readable from JS (mitigates XSS exfiltration).
+ * - SameSite=Strict: not sent on cross-site requests (mitigates CSWSH).
+ * - Path=/: scope to whole origin so /ws and /pty-session both see it.
+ * - Max-Age matches the TTL.
+ *
+ * Secure is intentionally omitted: the daemon binds to 127.0.0.1 over plain
+ * HTTP; setting Secure would prevent the browser from ever sending it back.
+ */
+export function buildPtySetCookie(token: string): string {
+  const maxAge = Math.floor(TTL_MS / 1000);
+  return `${PTY_COOKIE_NAME}=${token}; HttpOnly; SameSite=Strict; Path=/; Max-Age=${maxAge}`;
+}
+
+/** Clear the PTY session cookie. */
+export function buildPtyClearCookie(): string {
+  return `${PTY_COOKIE_NAME}=; HttpOnly; SameSite=Strict; Path=/; Max-Age=0`;
+}
+
+function pruneExpired(now: number): void {
+  let checked = 0;
+  for (const [token, session] of sessions) {
+    if (checked++ >= 20) break;
+    if (session.expiresAt <= now) sessions.delete(token);
+  }
+  while (sessions.size > MAX_SESSIONS) {
+    const first = sessions.keys().next().value;
+    if (!first) break;
+    sessions.delete(first);
+  }
+}
+
+// Test-only reset.
+export function __resetPtySessions(): void {
+  sessions.clear();
+}