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/storage/store_test.go

@@ -180,6 +180,63 @@ func TestLoadOrCreateConfigCreatesDefaultsOnFirstRun(t *testing.T) {
 	}
 }
 
+// TestLoadOrCreateConfigKeepsZeroTimeoutOnReload guards the "0 = no timeout"
+// setting against being normalized away when an existing gosentry.json is read
+// back. Loading must not treat 0 as a missing value.
+func TestLoadOrCreateConfigKeepsZeroTimeoutOnReload(t *testing.T) {
+	dir := t.TempDir()
+	paths := Paths{
+		AppDir:     dir,
+		ConfigPath: filepath.Join(dir, ConfigFileName),
+	}
+
+	// First call writes the defaults (DefaultTimeoutSeconds = 0) to disk.
+	if _, err := loadOrCreateConfig(paths); err != nil {
+		t.Fatal(err)
+	}
+	// Second call takes the "file exists" branch, where normalization runs.
+	reloaded, err := loadOrCreateConfig(paths)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if reloaded.DefaultTimeoutSeconds != 0 {
+		t.Errorf("reloaded DefaultTimeoutSeconds = %d, want 0 (no timeout)", reloaded.DefaultTimeoutSeconds)
+	}
+}
+
+// TestJobTimeoutRoundTripsThreeStates pins the on-disk encoding that keeps
+// "inherit" and "no timeout" distinguishable: nil is omitted entirely, while an
+// explicit 0 is written and read back as a set value.
+func TestJobTimeoutRoundTripsThreeStates(t *testing.T) {
+	jobs := []domain.Job{
+		{ID: 1, Name: "Inherit", TimeoutSeconds: nil},
+		{ID: 2, Name: "No timeout", TimeoutSeconds: domain.TimeoutSecondsPtr(0)},
+		{ID: 3, Name: "Own", TimeoutSeconds: domain.TimeoutSecondsPtr(45)},
+	}
+
+	data, err := json.Marshal(domain.JobsFile{Jobs: jobs})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := `"timeout_seconds":0`; !strings.Contains(string(data), want) {
+		t.Fatalf("explicit zero timeout should be written as %s:\n%s", want, data)
+	}
+
+	var got domain.JobsFile
+	if err := json.Unmarshal(data, &got); err != nil {
+		t.Fatal(err)
+	}
+	if got.Jobs[0].TimeoutSeconds != nil {
+		t.Errorf("unset timeout should stay nil, got %d", *got.Jobs[0].TimeoutSeconds)
+	}
+	if got.Jobs[1].TimeoutSeconds == nil || *got.Jobs[1].TimeoutSeconds != 0 {
+		t.Errorf("explicit zero timeout should survive the round trip, got %v", got.Jobs[1].TimeoutSeconds)
+	}
+	if got.Jobs[2].TimeoutSeconds == nil || *got.Jobs[2].TimeoutSeconds != 45 {
+		t.Errorf("per-job timeout should survive the round trip, got %v", got.Jobs[2].TimeoutSeconds)
+	}
+}
+
 func TestJobsJSONDoesNotPersistRuntimeNoise(t *testing.T) {
 	// Job carries only durable configuration; runtime state lives in
 	// domain.JobRuntime and is never marshalled. This guards against a future