feat(refinery): implement MR state transition validation (gt-h5n.3)

- Update MRStatus to use beads-style statuses (open, in_progress, closed)
- Add CloseReason enum for tracking why MRs were closed
- Implement ValidateTransition() to enforce valid state transitions:
  - open → in_progress (Engineer claims MR)
  - in_progress → closed (merge success or rejection)
  - in_progress → open (failure, reassign to worker)
  - open → closed (manual rejection)
  - closed → anything is blocked (immutable once closed)
- Add convenience methods: Claim(), Close(), Reopen(), SetStatus()
- Add status check methods: IsClosed(), IsOpen(), IsInProgress()
- Update ProcessMR and completeMR to use new state transition methods
- Update display code to handle new status values
- Add comprehensive tests for state transitions

Reference: docs/merge-queue-design.md#state-machine

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

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

internal/cmd/refinery.go

@@ -274,14 +274,27 @@ func runRefineryQueue(cmd *cobra.Command, args []string) error {
 			status = style.Bold.Render("[processing]")
 		} else {
 			switch item.MR.Status {
-			case refinery.MRPending:
-				status = style.Dim.Render("[pending]")
-			case refinery.MRMerged:
-				status = style.Bold.Render("[merged]")
-			case refinery.MRFailed:
-				status = style.Dim.Render("[failed]")
-			case refinery.MRSkipped:
-				status = style.Dim.Render("[skipped]")
+			case refinery.MROpen:
+				if item.MR.Error != "" {
+					status = style.Dim.Render("[needs-rework]")
+				} else {
+					status = style.Dim.Render("[pending]")
+				}
+			case refinery.MRInProgress:
+				status = style.Bold.Render("[processing]")
+			case refinery.MRClosed:
+				switch item.MR.CloseReason {
+				case refinery.CloseReasonMerged:
+					status = style.Bold.Render("[merged]")
+				case refinery.CloseReasonRejected:
+					status = style.Dim.Render("[rejected]")
+				case refinery.CloseReasonConflict:
+					status = style.Dim.Render("[conflict]")
+				case refinery.CloseReasonSuperseded:
+					status = style.Dim.Render("[superseded]")
+				default:
+					status = style.Dim.Render("[closed]")
+				}
 			}
 		}