feat: make per-job timeout 0 mean "no timeout" instead of inherit

A per-job timeout now has three distinct states: unset inherits the global
default, an explicit 0 means no timeout and does not inherit, and a positive
value is the per-job limit. Job.TimeoutSeconds became *int so unset and 0 stay
distinguishable in jobs.json.

Also fixes the global default, which could not persist a 0. loadOrCreateConfig
normalized DefaultTimeoutSeconds <= 0 back to 30 on every read of an existing
gosentry.json, so "no timeout" only held until the next restart. The field is
now written unconditionally (no omitempty) and read back as-is.

Existing jobs and configs are unaffected: a job with no timeout_seconds still
inherits, and a saved global default of 30 stays 30.

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

src/app/run.go

@@ -211,15 +211,16 @@ func (s *Service) effectiveOverlapPolicy(job *domain.Job) domain.OverlapPolicy {
 }
 
 // effectiveTimeout resolves the run timeout that actually governs a job: the
-// job's own TimeoutSeconds when positive, otherwise the global
-// Config.DefaultTimeoutSeconds. A non-positive Job.TimeoutSeconds means "inherit
-// the global default", which is why normalizeJob leaves 0 rather than
+// job's own TimeoutSeconds whenever it is set — including an explicit 0, which
+// means "no timeout" and deliberately does not inherit — otherwise the global
+// Config.DefaultTimeoutSeconds. A nil Job.TimeoutSeconds means "inherit the
+// global default", which is why normalizeJob leaves it nil rather than
 // backfilling the configured value. A resolved duration of 0 means no timeout;
 // runner.RunJob treats it as "run without a deadline". The caller must hold mu.
 func (s *Service) effectiveTimeout(job *domain.Job) time.Duration {
-	secs := job.TimeoutSeconds
-	if secs <= 0 {
-		secs = s.store.Config.DefaultTimeoutSeconds
+	secs := s.store.Config.DefaultTimeoutSeconds
+	if job.TimeoutSeconds != nil {
+		secs = *job.TimeoutSeconds
 	}
 	return time.Duration(secs) * time.Second
 }