fix: forward signals from parent to child process to prevent orphans - #29427

When the parent process (bootstrap or relaunch wrapper) receives a
termination signal such as SIGTERM or SIGHUP, the child process is now
notified instead of being silently orphaned.

Root cause: both index.ts (lightweight parent) and relaunch.ts
(relaunchAppInChildProcess) spawned a child with stdio:'inherit' but
never proxied signals. The parent either swallowed them with empty
handlers (index.ts) or let the default disposition kill it while the
child kept running (relaunch.ts).

Fix:
- Add SignalForwarder class in @google/gemini-cli-core that installs
  precise per-signal handlers, forwards them to the child via
  child.kill(), and escalates to SIGKILL after a configurable grace
  period (default 5s) for fatal signals (SIGTERM, SIGHUP, SIGQUIT).
- Integrate into relaunch.ts via the shared SignalForwarder class.
- Integrate into index.ts with inline forwarding (avoids importing
  the full core package to keep startup fast).
- Clean up handlers on child 'close' and 'error' events to prevent
  listener leaks across relaunch iterations.
- Add isChildProcess() helper to processUtils.ts.
- Update cleanup.ts JSDoc to document interaction with forwarding.
- Add GEMINI_CLI_NO_RELAUNCH to test-rig.ts clean env.
- Add build verification in build_package.js for the new module.

Tests:
- 31 unit tests for SignalForwarder class (install, remove, forwarding,
  escalation, idempotency, edge cases).
- 7 unit tests for signal forwarding in relaunch.test.ts (SIGTERM,
  SIGHUP, SIGUSR1/2, error handling, listener leak prevention).
- 3 integration tests spawning real processes verifying OS-level signal
  delivery (SIGTERM, SIGHUP, SIGUSR1).
- 3 unit tests for isChildProcess().
- 2 unit tests for cleanup signal handler coexistence.

Fixes google-gemini#25590