fix(runtime): cancel and reap exec watchdog timers

[AaronAbuUsama]

Summary

Successful container execs can leave their watchdog's sleep process running until the timeout. Cancel and reap that timer before returning, while retaining the outer watchdog wait added by #989.

The watchdog now terminates its owned timer with SIGKILL: rapid SIGTERM cancellation was unreliable under BusyBox and could delay a successful command until its full timeout. The command itself still receives SIGTERM at its deadline. Argument quoting, output, exit codes, process-group targeting, and the missing-sleep/setsid fallbacks remain intact.

Closes #1132.

Verification

• The original leak was reproduced with PostgreSQL as PID 1: three successful commands left three timers; the fix leaves none.
• A deterministic timer-cancellation regression fails on 96255e5 (a successful command took 5.3 seconds) and passes with this correction. The focused runtime file passes all 26 tests.
• New required Docker CI coverage runs the generated wrapper in BusyBox 1.37 and Debian/PostgreSQL: 2,000 rapid commands per image, cleanup, output and nonzero exit preservation, real deadlines, and PostgreSQL readiness afterward. Both cases passed locally.
• An additional 10,000 rapid BusyBox commands completed without a full-timeout stall or leftover timer.
• bun run test passed all 10 workspace tasks with Bun 1.3.14, including 4,786 adapter tests. Adapter and API typechecks and the structured-error audit passed.

The BusyBox test is part of the existing required Docker CI suite; an unavailable daemon fails that gate. Test containers use no network or published ports and are removed after each case. No API, schema, dependency, deployment-default, or init-setting change is included.

Checklist

• One process-lifecycle fix, with no unrelated changes.
• Regression demonstrated before the correction and verified afterward.
• Real BusyBox and PostgreSQL tests, focused tests, full workspace suite, and relevant typechecks passed.
• Changed code formatted without unrelated formatting changes.