feat(mail): add gt mail hook as alias for gt hook attach

Adds gt mail hook <mail-id> command that attaches a mail message to
the agents hook. This provides a more intuitive command path when
working with mail-based workflows.

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

internal/cmd/mail_hook.go

@@ -0,0 +1,58 @@
+package cmd
+
+import (
+	"github.com/spf13/cobra"
+)
+
+// Flags for mail hook command (mirror of hook command flags)
+var (
+	mailHookSubject string
+	mailHookMessage string
+	mailHookDryRun  bool
+	mailHookForce   bool
+)
+
+var mailHookCmd = &cobra.Command{
+	Use:   "hook <mail-id>",
+	Short: "Attach mail to your hook (alias for 'gt hook attach')",
+	Long: `Attach a mail message to your hook.
+
+This is an alias for 'gt hook attach <mail-id>'. It attaches the specified
+mail message to your hook so you can work on it.
+
+The hook is the "durability primitive" - work on your hook survives session
+restarts, context compaction, and handoffs.
+
+Examples:
+  gt mail hook msg-abc123                    # Attach mail to your hook
+  gt mail hook msg-abc123 -s "Fix the bug"   # With subject for handoff
+  gt mail hook msg-abc123 --force            # Replace existing incomplete work
+
+Related commands:
+  gt hook <bead>     # Attach any bead to your hook
+  gt hook status     # Show what's on your hook
+  gt unsling         # Remove work from hook`,
+	Args: cobra.ExactArgs(1),
+	RunE: runMailHook,
+}
+
+func init() {
+	mailHookCmd.Flags().StringVarP(&mailHookSubject, "subject", "s", "", "Subject for handoff mail (optional)")
+	mailHookCmd.Flags().StringVarP(&mailHookMessage, "message", "m", "", "Message for handoff mail (optional)")
+	mailHookCmd.Flags().BoolVarP(&mailHookDryRun, "dry-run", "n", false, "Show what would be done")
+	mailHookCmd.Flags().BoolVarP(&mailHookForce, "force", "f", false, "Replace existing incomplete hooked bead")
+
+	mailCmd.AddCommand(mailHookCmd)
+}
+
+// runMailHook attaches mail to the hook - delegates to the hook command's logic
+func runMailHook(cmd *cobra.Command, args []string) error {
+	// Copy flags to hook command's globals (they share the same functionality)
+	hookSubject = mailHookSubject
+	hookMessage = mailHookMessage
+	hookDryRun = mailHookDryRun
+	hookForce = mailHookForce
+
+	// Delegate to the hook command's run function
+	return runHook(cmd, args)
+}