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/format.go

@@ -103,12 +103,16 @@ func DisplayOverlapPolicy(job domain.Job, globalPolicy domain.OverlapPolicy) str
 }
 
 // DisplayTimeout formats a job's effective run timeout for the details panel.
-// When the job sets its own TimeoutSeconds it is shown as-is; when 0 (inherit),
-// the global default is shown with "(global default)" appended, mirroring
-// DisplayOverlapPolicy. A non-positive global default means no timeout at all.
+// When the job sets its own TimeoutSeconds it is shown as-is, with an explicit 0
+// rendered as "no timeout"; when unset (nil), the global default is shown with
+// "(global default)" appended, mirroring DisplayOverlapPolicy. A non-positive
+// global default means no timeout at all.
 func DisplayTimeout(job domain.Job, globalDefault int) string {
-	if job.TimeoutSeconds > 0 {
-		return fmt.Sprintf("%d s", job.TimeoutSeconds)
+	if job.TimeoutSeconds != nil {
+		if *job.TimeoutSeconds <= 0 {
+			return "no timeout"
+		}
+		return fmt.Sprintf("%d s", *job.TimeoutSeconds)
 	}
 	if globalDefault <= 0 {
 		return "no timeout (global default)"