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/cmd/mq_list.go

@@ -10,7 +10,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/gastown/internal/beads"
-	"github.com/steveyegge/gastown/internal/mrqueue"
+	"github.com/steveyegge/gastown/internal/refinery"
 	"github.com/steveyegge/gastown/internal/style"
 )
 
@@ -260,7 +260,7 @@ func outputJSON(data interface{}) error {
 	return enc.Encode(data)
 }
 
-// calculateMRScore computes the priority score for an MR using the mrqueue scoring function.
+// calculateMRScore computes the priority score for an MR using the refinery scoring function.
 // Higher scores mean higher priority (process first).
 func calculateMRScore(issue *beads.Issue, fields *beads.MRFields, now time.Time) float64 {
 	// Parse MR creation time
@@ -273,7 +273,7 @@ func calculateMRScore(issue *beads.Issue, fields *beads.MRFields, now time.Time)
 	}
 
 	// Build score input
-	input := mrqueue.ScoreInput{
+	input := refinery.ScoreInput{
 		Priority:    issue.Priority,
 		MRCreatedAt: mrCreatedAt,
 		Now:         now,
@@ -291,5 +291,5 @@ func calculateMRScore(issue *beads.Issue, fields *beads.MRFields, now time.Time)
 		}
 	}
 
-	return mrqueue.ScoreMRWithDefaults(input)
+	return refinery.ScoreMRWithDefaults(input)
 }

internal/cmd/refinery.go

@@ -6,7 +6,7 @@ import (
 	"os"
 
 	"github.com/spf13/cobra"
-	"github.com/steveyegge/gastown/internal/mrqueue"
+	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/refinery"
 	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/style"
@@ -540,19 +540,23 @@ func runRefineryClaim(cmd *cobra.Command, args []string) error {
 	mrID := args[0]
 	workerID := getWorkerID()
 
-	// Find the queue from current working directory
-	q, err := mrqueue.NewFromWorkdir(".")
+	// Find beads from current working directory
+	townRoot, err := workspace.FindFromCwdOrError()
 	if err != nil {
-		return fmt.Errorf("finding merge queue: %w", err)
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+	rigName, err := inferRigFromCwd(townRoot)
+	if err != nil {
+		return fmt.Errorf("could not determine rig: %w", err)
 	}
 
-	if err := q.Claim(mrID, workerID); err != nil {
-		if err == mrqueue.ErrNotFound {
-			return fmt.Errorf("MR %s not found in queue", mrID)
-		}
-		if err == mrqueue.ErrAlreadyClaimed {
-			return fmt.Errorf("MR %s is already claimed by another worker", mrID)
-		}
+	_, r, err := getRig(rigName)
+	if err != nil {
+		return err
+	}
+
+	eng := refinery.NewEngineer(r)
+	if err := eng.ClaimMR(mrID, workerID); err != nil {
 		return fmt.Errorf("claiming MR: %w", err)
 	}
 
@@ -563,12 +567,23 @@ func runRefineryClaim(cmd *cobra.Command, args []string) error {
 func runRefineryRelease(cmd *cobra.Command, args []string) error {
 	mrID := args[0]
 
-	q, err := mrqueue.NewFromWorkdir(".")
+	// Find beads from current working directory
+	townRoot, err := workspace.FindFromCwdOrError()
 	if err != nil {
-		return fmt.Errorf("finding merge queue: %w", err)
+		return fmt.Errorf("not in a Gas Town workspace: %w", err)
+	}
+	rigName, err := inferRigFromCwd(townRoot)
+	if err != nil {
+		return fmt.Errorf("could not determine rig: %w", err)
 	}
 
-	if err := q.Release(mrID); err != nil {
+	_, r, err := getRig(rigName)
+	if err != nil {
+		return err
+	}
+
+	eng := refinery.NewEngineer(r)
+	if err := eng.ReleaseMR(mrID); err != nil {
 		return fmt.Errorf("releasing MR: %w", err)
 	}
 
@@ -587,10 +602,35 @@ func runRefineryUnclaimed(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
-	q := mrqueue.New(r.Path)
-	unclaimed, err := q.ListUnclaimed()
+	// Query beads for merge-request issues without assignee
+	b := beads.New(r.Path)
+	issues, err := b.List(beads.ListOptions{
+		Status:   "open",
+		Label:    "gt:merge-request",
+		Priority: -1,
+	})
 	if err != nil {
-		return fmt.Errorf("listing unclaimed MRs: %w", err)
+		return fmt.Errorf("listing merge requests: %w", err)
+	}
+
+	// Filter for unclaimed (no assignee)
+	var unclaimed []*refinery.MRInfo
+	for _, issue := range issues {
+		if issue.Assignee != "" {
+			continue
+		}
+		fields := beads.ParseMRFields(issue)
+		if fields == nil {
+			continue
+		}
+		mr := &refinery.MRInfo{
+			ID:       issue.ID,
+			Branch:   fields.Branch,
+			Target:   fields.Target,
+			Worker:   fields.Worker,
+			Priority: issue.Priority,
+		}
+		unclaimed = append(unclaimed, mr)
 	}
 
 	// JSON output