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/test/process-liveness-windows.test.ts

@@ -100,12 +100,16 @@ describe('process liveness probe (Windows terminal-agent leak)', () => {
       const pid = spawnTerminalAgent({
         stateFile: path.join(tmpDir, 'state.json'),
         serverPort: 12345,
+        ownerPid: process.pid,
         cwd: tmpDir,
         scriptPath: script,
       });
       expect(pid).toBe(4242);
       expect(captured).not.toBeNull();
       expect(captured.windowsHide).toBe(true);
+      // Owner-PID lifetime tie (#2019): the agent polls this and exits when
+      // its owning browse server dies, so it can't be adopted by PID 1.
+      expect(captured.env.BROWSE_OWNER_PID).toBe(String(process.pid));
       // Detached background daemon — must not inherit a terminal either.
       expect(captured.stdio).toEqual(['ignore', 'ignore', 'ignore']);
     } finally {