fix(cron): reload store on external file change; only save when state changes

Two related bugs caused cron jobs added via the CLI to be silently lost
while picoclaw was running:

1. checkJobs() called saveStoreUnsafe() unconditionally every second,
   overwriting the file with the running service's in-memory state. Any
   job written by the CLI was clobbered within at most one tick.

2. The running service never reloaded the store from disk, so CLI-added
   jobs were invisible in memory and would never execute even if the file
   was not overwritten.

Fix: track the store file's mtime in fileModTime (updated after every
load and save). At the start of each checkJobs() tick, stat the file; if
its mtime is newer than fileModTime, reload from disk. Only call
saveStoreUnsafe() when there are actually due jobs — no state changes
means no write, which stops the clobbering entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/cron/service.go

@@ -66,6 +66,7 @@ type CronService struct {
 	running     bool
 	stopChan    chan struct{}
 	gronx       *gronx.Gronx
+	fileModTime time.Time // mtime of store file at last load or save
 }
 
 func NewCronService(storePath string, onJob JobHandler) *CronService {
@@ -140,6 +141,13 @@ func (cs *CronService) checkJobs() {
 		return
 	}
 
+	// Reload from disk if the file was modified externally (e.g. by the CLI).
+	if info, err := os.Stat(cs.storePath); err == nil && info.ModTime().After(cs.fileModTime) {
+		if err := cs.loadStore(); err != nil {
+			log.Printf("[cron] failed to reload store: %v", err)
+		}
+	}
+
 	now := time.Now().UnixMilli()
 	var dueJobIDs []string
 
@@ -151,7 +159,10 @@ func (cs *CronService) checkJobs() {
 		}
 	}
 
-	// Reset next run for due jobs before unlocking to avoid duplicate execution.
+	// Only persist state when there are due jobs to avoid clobbering
+	// concurrent writes from the CLI or other processes.
+	if len(dueJobIDs) > 0 {
+		// Reset next run before unlocking to prevent duplicate execution.
 		dueMap := make(map[string]bool, len(dueJobIDs))
 		for _, jobID := range dueJobIDs {
 			dueMap[jobID] = true
@@ -165,6 +176,7 @@ func (cs *CronService) checkJobs() {
 		if err := cs.saveStoreUnsafe(); err != nil {
 			log.Printf("[cron] failed to save store: %v", err)
 		}
+	}
 
 	cs.mu.Unlock()
 
@@ -347,7 +359,13 @@ func (cs *CronService) loadStore() error {
 		return err
 	}
 
-	return json.Unmarshal(data, cs.store)
+	if err := json.Unmarshal(data, cs.store); err != nil {
+		return err
+	}
+	if info, err := os.Stat(cs.storePath); err == nil {
+		cs.fileModTime = info.ModTime()
+	}
+	return nil
 }
 
 func (cs *CronService) saveStoreUnsafe() error {
@@ -357,7 +375,13 @@ func (cs *CronService) saveStoreUnsafe() error {
 	}
 
 	// Use unified atomic write utility with explicit sync for flash storage reliability.
-	return fileutil.WriteFileAtomic(cs.storePath, data, 0o600)
+	if err := fileutil.WriteFileAtomic(cs.storePath, data, 0o600); err != nil {
+		return err
+	}
+	if info, err := os.Stat(cs.storePath); err == nil {
+		cs.fileModTime = info.ModTime()
+	}
+	return nil
 }
 
 func (cs *CronService) AddJob(