Implement convoy --notify flag notification (gt-5tipl)

The --notify flag on gt convoy create was captured but never used.
This fix:

1. Stores the notify address in a bd slot when convoy is created
2. Adds 'gt convoy notify <convoy-id>' command that:
   - Reads the notify slot
   - Sends mail to the configured address with convoy completion details
   - Does nothing gracefully if no notification was configured

The convoy-cleanup formula can now call 'gt convoy notify {{convoy}}'
to trigger the configured notification when a convoy completes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/convoy.go

@@ -92,6 +92,23 @@ Examples:
 	RunE: runConvoyList,
 }
 
+var convoyNotifyCmd = &cobra.Command{
+	Use:   "notify <convoy-id>",
+	Short: "Send completion notification for a convoy",
+	Long: `Send the completion notification for a convoy.
+
+If the convoy was created with --notify, sends mail to that address.
+Otherwise does nothing (convoy had no notification configured).
+
+This is typically called by the convoy-cleanup formula when a convoy
+completes. It can also be called manually to resend a notification.
+
+Examples:
+  gt convoy notify hq-abc123`,
+	Args: cobra.ExactArgs(1),
+	RunE: runConvoyNotify,
+}
+
 func init() {
 	// Create flags
 	convoyCreateCmd.Flags().StringVar(&convoyMolecule, "molecule", "", "Associated molecule ID")
@@ -108,6 +125,7 @@ func init() {
 	convoyCmd.AddCommand(convoyCreateCmd)
 	convoyCmd.AddCommand(convoyStatusCmd)
 	convoyCmd.AddCommand(convoyListCmd)
+	convoyCmd.AddCommand(convoyNotifyCmd)
 
 	rootCmd.AddCommand(convoyCmd)
 }
@@ -171,6 +189,16 @@ func runConvoyCreate(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("convoy created but no ID returned")
 	}
 
+	// Store notify address in slot if specified (for convoy-cleanup to read)
+	if convoyNotify != "" {
+		slotArgs := []string{"slot", "set", convoyID, "notify", convoyNotify}
+		slotCmd := exec.Command("bd", slotArgs...)
+		slotCmd.Dir = townBeads
+		if err := slotCmd.Run(); err != nil {
+			style.PrintWarning("couldn't set notify slot: %v", err)
+		}
+	}
+
 	// Add 'tracks' relations for each tracked issue
 	trackedCount := 0
 	for _, issueID := range trackedIssues {
@@ -413,6 +441,95 @@ func runConvoyList(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+func runConvoyNotify(cmd *cobra.Command, args []string) error {
+	convoyID := args[0]
+
+	townBeads, err := getTownBeadsDir()
+	if err != nil {
+		return err
+	}
+
+	// Get convoy details
+	showArgs := []string{"show", convoyID, "--json"}
+	showCmd := exec.Command("bd", showArgs...)
+	showCmd.Dir = townBeads
+	var stdout bytes.Buffer
+	showCmd.Stdout = &stdout
+
+	if err := showCmd.Run(); err != nil {
+		return fmt.Errorf("convoy '%s' not found", convoyID)
+	}
+
+	// Parse convoy data
+	var convoys []struct {
+		ID          string `json:"id"`
+		Title       string `json:"title"`
+		Status      string `json:"status"`
+		Description string `json:"description"`
+	}
+	if err := json.Unmarshal(stdout.Bytes(), &convoys); err != nil {
+		return fmt.Errorf("parsing convoy data: %w", err)
+	}
+
+	if len(convoys) == 0 {
+		return fmt.Errorf("convoy '%s' not found", convoyID)
+	}
+
+	convoy := convoys[0]
+
+	// Get notify address from slot
+	slotArgs := []string{"slot", "get", convoyID, "notify"}
+	slotCmd := exec.Command("bd", slotArgs...)
+	slotCmd.Dir = townBeads
+	var slotStdout bytes.Buffer
+	slotCmd.Stdout = &slotStdout
+
+	if err := slotCmd.Run(); err != nil {
+		// No notify slot means no notification configured
+		fmt.Printf("%s No notification configured for convoy %s\n", style.Dim.Render("○"), convoyID)
+		return nil
+	}
+
+	notifyAddr := strings.TrimSpace(slotStdout.String())
+	if notifyAddr == "" {
+		fmt.Printf("%s No notification configured for convoy %s\n", style.Dim.Render("○"), convoyID)
+		return nil
+	}
+
+	// Get tracked issue stats
+	tracked := getTrackedIssues(townBeads, convoyID)
+	completed := 0
+	for _, t := range tracked {
+		if t.Status == "closed" {
+			completed++
+		}
+	}
+
+	// Build notification message
+	subject := fmt.Sprintf("Convoy complete: %s", convoy.Title)
+	body := fmt.Sprintf(`Convoy %s has completed.
+
+Title: %s
+Status: %s
+Tracked: %d issues (%d completed)
+
+View details: bd show %s`,
+		convoyID, convoy.Title, convoy.Status, len(tracked), completed, convoyID)
+
+	// Send mail notification
+	mailArgs := []string{"mail", "send", notifyAddr, "-s", subject, "-m", body}
+	mailCmd := exec.Command("gt", mailArgs...)
+	mailCmd.Stdout = os.Stdout
+	mailCmd.Stderr = os.Stderr
+
+	if err := mailCmd.Run(); err != nil {
+		return fmt.Errorf("sending notification to %s: %w", notifyAddr, err)
+	}
+
+	fmt.Printf("%s Notification sent to %s\n", style.Bold.Render("✓"), notifyAddr)
+	return nil
+}
+
 func formatConvoyStatus(status string) string {
 	switch status {
 	case "open":