fix(terminal-agent): tie agent lifetime to its owning browse server PID

The terminal agent is intentionally detached so it survives the
short-lived CLI launcher, but its real owner is the persistent browse
server. If that server crashed or was killed before running normal
shutdown, the agent was adopted by PID 1 and lived forever (#2019).

spawnTerminalAgent now requires an ownerPid and exports it to the agent
as BROWSE_OWNER_PID; all three spawn sites pass the server PID (cli.ts
cold-start, cli.ts supervisor respawn, server.ts watchdog). The agent
polls the owner with signal 0 every 15s (GSTACK_TERMINAL_OWNER_WATCHDOG_MS
to tune) on an unref'd timer and, when the owner disappears, exits
through the SAME cleanup path as an intentional SIGTERM shutdown — now
re-entrancy-guarded and also removing the terminal-internal-token file
alongside the port file and agent record.

Runtime test spawns a real agent tied to a throwaway owner process,
kills the owner, and asserts the agent exits and its discovery files
(terminal-agent-pid, terminal-port) are gone.

Reconciled with the watchdog commit's spawnTerminalAgent contract test
(process-liveness-windows.test.ts now passes ownerPid and pins the
BROWSE_OWNER_PID env forwarding).

Closes #2019.

Contributed by @csarigoz (PR #2530).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/cli.ts

@@ -1130,6 +1130,7 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
         const newPid = spawnTerminalAgent({
           stateFile: config.stateFile,
           serverPort: newState.port,
+          ownerPid: newState.pid,
           cwd: config.projectDir,
         });
         if (newPid) {
@@ -1222,6 +1223,7 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
           spawnTerminalAgent({
             stateFile: config.stateFile,
             serverPort: respawned.port,
+            ownerPid: respawned.pid,
             cwd: config.projectDir,
           });
         } catch (err: any) {

browse/src/server.ts

@@ -1573,6 +1573,7 @@ export function buildFetchHandler(cfg: ServerConfig): ServerHandle {
         const pid = spawnTerminalAgent({
           stateFile: cfg.config.stateFile,
           serverPort: cfg.browsePort,
+          ownerPid: process.pid,
           cwd: cfg.config.projectDir,
         });
         if (pid) {

browse/src/terminal-agent-control.ts

@@ -48,12 +48,13 @@ export function resolveTerminalAgentScript(searchHints: { metaDir?: string; exec
  *
  * Used by both the CLI cold-start path (cli.ts) and the v1.44 watchdog in
  * server.ts. Centralizing here removes a copy-paste between them and means
- * future spawn-env additions (e.g. BROWSE_OWNER_PID for the generation
- * counter rollout) land in one place.
+ * spawn-env additions (BROWSE_OWNER_PID being the first) land in one place.
  */
 export function spawnTerminalAgent(opts: {
   stateFile: string;
   serverPort: number;
+  /** PID of the browse server that owns this agent. */
+  ownerPid: number;
   cwd?: string;
   /** Optional extra env vars to add to the agent's process env. */
   extraEnv?: Record<string, string>;
@@ -74,6 +75,7 @@ export function spawnTerminalAgent(opts: {
       ...process.env,
       BROWSE_STATE_FILE: opts.stateFile,
       BROWSE_SERVER_PORT: String(opts.serverPort),
+      BROWSE_OWNER_PID: String(opts.ownerPid),
       ...(opts.extraEnv || {}),
     },
     stdio: ['ignore', 'ignore', 'ignore'],

browse/src/terminal-agent.ts

@@ -30,6 +30,11 @@ import { writeAgentRecord, clearAgentRecord } from './terminal-agent-control';
 const STATE_FILE = process.env.BROWSE_STATE_FILE || path.join(process.env.HOME || '/tmp', '.gstack', 'browse.json');
 const PORT_FILE = path.join(path.dirname(STATE_FILE), 'terminal-port');
 const BROWSE_SERVER_PORT = parseInt(process.env.BROWSE_SERVER_PORT || '0', 10);
+const BROWSE_OWNER_PID = parseInt(process.env.BROWSE_OWNER_PID || '0', 10);
+const OWNER_WATCHDOG_MS = parseInt(
+  process.env.GSTACK_TERMINAL_OWNER_WATCHDOG_MS || '15000',
+  10,
+);
 const EXTENSION_ID = process.env.BROWSE_EXTENSION_ID || ''; // optional: tighten Origin check
 const INTERNAL_TOKEN = crypto.randomBytes(32).toString('base64url'); // shared with parent server via env at spawn
 /**
@@ -987,13 +992,33 @@ function main() {
   console.log(`[terminal-agent] listening on 127.0.0.1:${port} pid=${process.pid} gen=${CURRENT_GEN}`);
 
   // Cleanup port file + agent record on exit.
+  let cleaningUp = false;
   const cleanup = () => {
+    if (cleaningUp) return;
+    cleaningUp = true;
     safeUnlink(PORT_FILE);
+    safeUnlink(INTERNAL_TOKEN_FILE);
     clearAgentRecord(dir);
     process.exit(0);
   };
   process.on('SIGTERM', cleanup);
   process.on('SIGINT', cleanup);
+
+  // The terminal agent is intentionally detached so it survives the short-lived
+  // CLI launcher, but its real owner is the persistent browse server. If that
+  // server crashes or is killed before running normal shutdown, the agent would
+  // otherwise be adopted by PID 1 and live forever. Poll the server PID and use
+  // the same cleanup path as an intentional shutdown when it disappears.
+  if (BROWSE_OWNER_PID > 0) {
+    const ownerWatchdog = setInterval(() => {
+      try {
+        process.kill(BROWSE_OWNER_PID, 0);
+      } catch {
+        cleanup();
+      }
+    }, OWNER_WATCHDOG_MS);
+    (ownerWatchdog as any)?.unref?.();
+  }
 }
 
 // Export the internal token so cli.ts can pass the SAME value to the parent