feat(mq): auto-cleanup polecats after MR submission

When `gt mq submit` is run from a polecat work branch (polecat/<worker>/<issue>),
it now automatically triggers polecat shutdown after submitting the MR. The
polecat sends a lifecycle request to its Witness and waits for termination.

This eliminates the need for polecats to manually run `gt handoff --shutdown`
after completing work - they can just run `gt mq submit` and the cleanup
happens automatically.

Added `--no-cleanup` flag to disable auto-cleanup when needed (e.g., for
submitting multiple MRs or continuing work).

Closes gt-tca

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

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

internal/cmd/handoff.go

@@ -436,8 +436,8 @@ func submitMRForPolecat() error {
 		return nil
 	}
 
-	// Run gt mq submit
-	submitCmd := exec.Command("gt", "mq", "submit")
+	// Run gt mq submit --no-cleanup (handoff manages lifecycle itself)
+	submitCmd := exec.Command("gt", "mq", "submit", "--no-cleanup")
 	submitOutput, err := submitCmd.CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("%s", strings.TrimSpace(string(submitOutput)))