refactor(mrqueue): remove mrqueue package, use beads for MRs (gt-dqi)

Remove the mrqueue side-channel from gastown. The merge queue now uses
beads merge-request wisps exclusively, not parallel .beads/mq/*.json files.

Changes:
- Delete internal/mrqueue/ package (~830 lines removed)
- Move scoring logic to internal/refinery/score.go
- Update Refinery engineer to query beads via ReadyWithType("merge-request")
- Add MRInfo struct to replace mrqueue.MR
- Add ClaimMR/ReleaseMR methods using beads assignee field
- Update HandleMergeReady to not create duplicate queue entries
- Update gt refinery commands (claim, release, unclaimed) to use beads
- Stub out MQEventSource (no longer needed)

The Refinery now:
- Lists MRs via beads.ReadyWithType("merge-request")
- Claims via beads.Update(..., {Assignee: worker})
- Closes via beads.CloseWithReason("merged", mrID)
- Blocks on conflicts via beads.AddDependency(mrID, taskID)

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

internal/protocol/refinery_handlers.go

@@ -4,14 +4,13 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"time"
 
 	"github.com/steveyegge/gastown/internal/mail"
-	"github.com/steveyegge/gastown/internal/mrqueue"
 )
 
 // DefaultRefineryHandler provides the default implementation for Refinery protocol handlers.
-// It receives MERGE_READY messages from the Witness and adds work to the merge queue.
+// It receives MERGE_READY messages from the Witness and acknowledges verified work.
+// Note: The Refinery now queries beads directly for merge requests (via ReadyWithType).
 type DefaultRefineryHandler struct {
 	// Rig is the name of the rig this refinery processes.
 	Rig string
@@ -19,9 +18,6 @@ type DefaultRefineryHandler struct {
 	// WorkDir is the working directory for operations.
 	WorkDir string
 
-	// Queue is the merge request queue.
-	Queue *mrqueue.Queue
-
 	// Router is used to send mail messages.
 	Router *mail.Router
 
@@ -34,7 +30,6 @@ func NewRefineryHandler(rig, workDir string) *DefaultRefineryHandler {
 	return &DefaultRefineryHandler{
 		Rig:     rig,
 		WorkDir: workDir,
-		Queue:   mrqueue.New(workDir),
 		Router:  mail.NewRouter(workDir),
 		Output:  os.Stdout,
 	}
@@ -46,10 +41,10 @@ func (h *DefaultRefineryHandler) SetOutput(w io.Writer) {
 }
 
 // HandleMergeReady handles a MERGE_READY message from Witness.
-// When a polecat's work is verified and ready, the Refinery:
-// 1. Validates the merge request
-// 2. Adds it to the merge queue
-// 3. Acknowledges receipt
+// When a polecat's work is verified and ready, the Refinery acknowledges receipt.
+//
+// NOTE: The merge-request bead is created by `gt done`, so we no longer need
+// to add to the mrqueue here. The Refinery queries beads directly for ready MRs.
 func (h *DefaultRefineryHandler) HandleMergeReady(payload *MergeReadyPayload) error {
 	_, _ = fmt.Fprintf(h.Output, "[Refinery] MERGE_READY received for polecat %s\n", payload.Polecat)
 	_, _ = fmt.Fprintf(h.Output, "  Branch: %s\n", payload.Branch)
@@ -64,25 +59,10 @@ func (h *DefaultRefineryHandler) HandleMergeReady(payload *MergeReadyPayload) er
 		return fmt.Errorf("missing polecat in MERGE_READY payload")
 	}
 
-	// Create merge request (ID is generated by Submit if empty)
-	mr := &mrqueue.MR{
-		Branch:      payload.Branch,
-		Worker:      payload.Polecat,
-		SourceIssue: payload.Issue,
-		Target:      "main", // Default target, could be passed in payload
-		Rig:         payload.Rig,
-		Title:       fmt.Sprintf("Merge %s work on %s", payload.Polecat, payload.Issue),
-		CreatedAt:   time.Now(),
-	}
-
-	// Add to queue
-	if err := h.Queue.Submit(mr); err != nil {
-		_, _ = fmt.Fprintf(h.Output, "[Refinery] Error adding to queue: %v\n", err)
-		return fmt.Errorf("failed to add merge request to queue: %w", err)
-	}
-
-	_, _ = fmt.Fprintf(h.Output, "[Refinery] ✓ Added to merge queue: %s\n", mr.ID)
-	_, _ = fmt.Fprintf(h.Output, "  Queue length: %d\n", h.Queue.Count())
+	// The merge-request bead is created by `gt done` with gt:merge-request label.
+	// The Refinery queries beads directly via ReadyWithType("merge-request").
+	// No need to add to mrqueue - that was a duplicate tracking file.
+	_, _ = fmt.Fprintf(h.Output, "[Refinery] ✓ Work verified - Refinery will pick up MR via beads query\n")
 
 	return nil
 }