Address issues found by code review:
1. Bug: timeout/signal-killed child reported as 'completed' with exit
code 0 because close handler ignored the signal parameter. Now
treats any signal termination as timed_out.
2. Bug: cancelled job gets overwritten by watcher on child exit. The
watcher now re-reads the job file before writing and skips if the
status has been changed to 'cancelled'.
3. Inconsistency: watcher path skipped stderr noise filtering. Added
filterStderrNoise to the watcher (duplicated from execute.ts to
keep the watcher self-contained).
4. getJobResult now guards against missing result field instead of
using non-null assertion.
stefano
0bea1c590d