feat: implement gt mq retry command for failed merge requests

Add 'gt mq retry <rig> <mr-id>' command to retry failed MRs:
- Added GetMR() and Retry() methods to refinery.Manager
- Added RegisterMR() for persistent MR tracking
- Added PendingMRs field to Refinery state
- Created new mq.go command file with retry subcommand
- Support --now flag for immediate processing
- Added comprehensive tests for retry functionality

Closes gt-svi.4

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

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

internal/cmd/mq.go

@@ -0,0 +1,92 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/refinery"
+	"github.com/steveyegge/gastown/internal/style"
+)
+
+// mq command flags
+var (
+	mqRetryNow bool
+)
+
+var mqCmd = &cobra.Command{
+	Use:   "mq",
+	Short: "Merge queue operations",
+	Long: `Manage the merge queue for a rig.
+
+The merge queue tracks work from polecats waiting to be merged.
+Use these commands to view, retry, and manage merge requests.`,
+}
+
+var mqRetryCmd = &cobra.Command{
+	Use:   "retry <rig> <mr-id>",
+	Short: "Retry a failed merge request",
+	Long: `Retry a failed merge request.
+
+Resets a failed MR so it can be processed again by the refinery.
+The MR must be in a failed state (open with an error).
+
+Examples:
+  gt mq retry gastown gt-mr-abc123
+  gt mq retry gastown gt-mr-abc123 --now`,
+	Args: cobra.ExactArgs(2),
+	RunE: runMQRetry,
+}
+
+func init() {
+	// Retry flags
+	mqRetryCmd.Flags().BoolVar(&mqRetryNow, "now", false, "Immediately process instead of waiting for refinery loop")
+
+	// Add subcommands
+	mqCmd.AddCommand(mqRetryCmd)
+
+	rootCmd.AddCommand(mqCmd)
+}
+
+func runMQRetry(cmd *cobra.Command, args []string) error {
+	rigName := args[0]
+	mrID := args[1]
+
+	mgr, _, err := getRefineryManager(rigName)
+	if err != nil {
+		return err
+	}
+
+	// Get the MR first to show info
+	mr, err := mgr.GetMR(mrID)
+	if err != nil {
+		if err == refinery.ErrMRNotFound {
+			return fmt.Errorf("merge request '%s' not found in rig '%s'", mrID, rigName)
+		}
+		return fmt.Errorf("getting merge request: %w", err)
+	}
+
+	// Show what we're retrying
+	fmt.Printf("Retrying merge request: %s\n", mrID)
+	fmt.Printf("  Branch: %s\n", mr.Branch)
+	fmt.Printf("  Worker: %s\n", mr.Worker)
+	if mr.Error != "" {
+		fmt.Printf("  Previous error: %s\n", style.Dim.Render(mr.Error))
+	}
+
+	// Perform the retry
+	if err := mgr.Retry(mrID, mqRetryNow); err != nil {
+		if err == refinery.ErrMRNotFailed {
+			return fmt.Errorf("merge request '%s' has not failed (status: %s)", mrID, mr.Status)
+		}
+		return fmt.Errorf("retrying merge request: %w", err)
+	}
+
+	if mqRetryNow {
+		fmt.Printf("%s Merge request processed\n", style.Bold.Render("✓"))
+	} else {
+		fmt.Printf("%s Merge request queued for retry\n", style.Bold.Render("✓"))
+		fmt.Printf("  %s\n", style.Dim.Render("Will be processed on next refinery cycle"))
+	}
+
+	return nil
+}