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/refinery/engineer_test.go

@@ -206,9 +206,6 @@ func TestNewEngineer(t *testing.T) {
 	if e.config == nil {
 		t.Error("expected config to be initialized with defaults")
 	}
-	if e.stopCh == nil {
-		t.Error("expected stopCh to be initialized")
-	}
 }
 
 func TestEngineer_DeleteMergedBranchesConfig(t *testing.T) {