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/refinery/manager.go

@@ -16,7 +16,6 @@ import (
 	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/mail"
-	"github.com/steveyegge/gastown/internal/mrqueue"
 	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/runtime"
 	"github.com/steveyegge/gastown/internal/session"
@@ -358,7 +357,7 @@ func (m *Manager) calculateIssueScore(issue *beads.Issue, now time.Time) float64
 	}
 
 	// Build score input
-	input := mrqueue.ScoreInput{
+	input := ScoreInput{
 		Priority:    issue.Priority,
 		MRCreatedAt: mrCreatedAt,
 		Now:         now,
@@ -376,7 +375,7 @@ func (m *Manager) calculateIssueScore(issue *beads.Issue, now time.Time) float64
 		}
 	}
 
-	return mrqueue.ScoreMRWithDefaults(input)
+	return ScoreMRWithDefaults(input)
 }
 
 // issueToMR converts a beads issue to a MergeRequest.