feat(refinery): implement merge failure handling with labels and notifications

Add comprehensive failure handling for merge queue:
- Add FailureType enum with conflict, tests_fail, build_fail, flaky_test, push_fail
- Add handleFailure function that updates beads issue with labels and assignee
- Add notifyWorkerFailure for type-specific failure notifications
- Add retry logic for flaky tests (configurable via retry_flaky_tests)
- Add pushWithRetry with exponential backoff for transient push failures
- Add label support to beads UpdateOptions (AddLabels, RemoveLabels, SetLabels)

Failure actions by type:
- conflict: needs-rebase label, assign to worker
- tests_fail/build_fail: needs-fix label, assign to worker
- flaky_test: retry once, then treat as tests_fail
- push_fail: retry with backoff, needs-retry label

Closes gt-3x1.4

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

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

internal/beads/beads.go

@@ -75,11 +75,14 @@ type CreateOptions struct {
 
 // UpdateOptions specifies options for updating an issue.
 type UpdateOptions struct {
-	Title       *string
-	Status      *string
-	Priority    *int
-	Description *string
-	Assignee    *string
+	Title        *string
+	Status       *string
+	Priority     *int
+	Description  *string
+	Assignee     *string
+	AddLabels    []string // Labels to add
+	RemoveLabels []string // Labels to remove
+	SetLabels    []string // Labels to set (replaces all existing)
 }
 
 // SyncStatus represents the sync status of the beads repository.
@@ -277,6 +280,19 @@ func (b *Beads) Update(id string, opts UpdateOptions) error {
 	if opts.Assignee != nil {
 		args = append(args, "--assignee="+*opts.Assignee)
 	}
+	// Label operations: set-labels replaces all, otherwise use add/remove
+	if len(opts.SetLabels) > 0 {
+		for _, label := range opts.SetLabels {
+			args = append(args, "--set-labels="+label)
+		}
+	} else {
+		for _, label := range opts.AddLabels {
+			args = append(args, "--add-label="+label)
+		}
+		for _, label := range opts.RemoveLabels {
+			args = append(args, "--remove-label="+label)
+		}
+	}
 
 	_, err := b.run(args...)
 	return err