Remove stubbed ProcessMRFromQueue and mrqueue package (gt-u4fh)

The mrqueue package was aspirational 'beads-based MR automation' that was
never completed. The Refinery agent is prompt-driven and uses beads/mail
for coordination, not a Go-based polling queue.

Removed:
- internal/mrqueue/mrqueue.go (entire package)
- internal/cmd/mq_migrate.go (migration command for mrqueue)
- mrqueue submission from done.go and mq_submit.go
- Engineer.ProcessMRFromQueue() and related queue handlers
- Engineer.Run(), Stop(), processOnce() methods
- mrQueue field and stopCh from Engineer struct
- stopCh assertion from TestNewEngineer

Kept:
- Bead creation for merge-requests (audit record)
- Engineer struct and NewEngineer for potential future use
- Engineer.ProcessMR() (works with beads.Issue)
- Manager.ProcessMR() which is the working implementation

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

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

internal/cmd/done.go

@@ -10,7 +10,6 @@ import (
 	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/git"
 	"github.com/steveyegge/gastown/internal/mail"
-	"github.com/steveyegge/gastown/internal/mrqueue"
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/workspace"
 )
@@ -164,28 +163,6 @@ func runDone(cmd *cobra.Command, args []string) error {
 		}
 		mrID = mrIssue.ID
 
-		// Also submit to mrqueue so refinery can process it
-		// The mrqueue is the work queue the refinery polls; beads are the audit record
-		mq, err := mrqueue.NewFromWorkdir(cwd)
-		if err != nil {
-			// Non-fatal: bead was created, just warn about queue
-			style.PrintWarning("could not access merge queue: %v", err)
-		} else {
-			mqEntry := &mrqueue.MR{
-				ID:          mrID,
-				Branch:      branch,
-				Target:      target,
-				SourceIssue: issueID,
-				Worker:      worker,
-				Rig:         rigName,
-				Title:       title,
-				Priority:    priority,
-			}
-			if err := mq.Submit(mqEntry); err != nil {
-				style.PrintWarning("could not submit to merge queue: %v", err)
-			}
-		}
-
 		// Success output
 		fmt.Printf("%s Work submitted to merge queue\n", style.Bold.Render("✓"))
 		fmt.Printf("  MR ID: %s\n", style.Bold.Render(mrID))