fix(refinery): serialize all pushes to main via merge slot

Add merge slot acquisition in doMerge() before pushing to origin/main.
This prevents a race condition where the normal merge path and a
conflict-resolution polecat can push to main concurrently, causing
non-fast-forward errors.

The fix:
- Acquires the merge slot before pushing (non-blocking)
- If slot is held by another process, returns failure so MR retries later
- Releases the slot after push completes (success or failure) via defer

This ensures all pushes to main are serialized, regardless of whether
they come from the normal merge queue or conflict resolution.

Fixes: gastownhallgh-594

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

Author: furiosa

internal/refinery/engineer.go

@@ -352,6 +352,44 @@ func (e *Engineer) doMerge(ctx context.Context, branch, target, sourceIssue stri
 	}
 
 	// Step 7: Push to origin
+	// === MERGE SLOT GATE: Serialize all pushes to main ===
+	// Acquire the merge slot to prevent racing with conflict-resolution polecats.
+	// This fixes the race condition where both the normal merge path and a
+	// conflict-resolution polecat can push to main concurrently (gh-594).
+	holder := e.rig.Name + "/refinery"
+	_, slotErr := e.beads.MergeSlotEnsureExists()
+	if slotErr != nil {
+		_, _ = fmt.Fprintf(e.output, "[Engineer] Warning: could not ensure merge slot: %v\n", slotErr)
+		// Continue anyway - slot is optional for backwards compatibility
+	}
+	slotStatus, slotErr := e.beads.MergeSlotAcquire(holder, false)
+	if slotErr != nil {
+		_, _ = fmt.Fprintf(e.output, "[Engineer] Warning: could not acquire merge slot: %v\n", slotErr)
+		// Continue anyway - slot is optional for backwards compatibility
+	} else if !slotStatus.Available && slotStatus.Holder != "" && slotStatus.Holder != holder {
+		// Slot is held by someone else (likely a conflict-resolution polecat)
+		// Return failure so MR stays in queue and retries after slot is released
+		_, _ = fmt.Fprintf(e.output, "[Engineer] Merge slot held by %s - deferring push\n", slotStatus.Holder)
+		return ProcessResult{
+			Success: false,
+			Error:   fmt.Sprintf("merge slot held by %s - will retry when released", slotStatus.Holder),
+		}
+	} else {
+		_, _ = fmt.Fprintf(e.output, "[Engineer] Acquired merge slot for push\n")
+	}
+	// Release the slot after push completes (success or failure)
+	defer func() {
+		if releaseErr := e.beads.MergeSlotRelease(holder); releaseErr != nil {
+			// Only log if it's a real error (not "slot not held")
+			errStr := releaseErr.Error()
+			if !strings.Contains(errStr, "not held") && !strings.Contains(errStr, "not found") {
+				_, _ = fmt.Fprintf(e.output, "[Engineer] Warning: failed to release merge slot: %v\n", releaseErr)
+			}
+		} else {
+			_, _ = fmt.Fprintf(e.output, "[Engineer] Released merge slot\n")
+		}
+	}()
+
 	_, _ = fmt.Fprintf(e.output, "[Engineer] Pushing to origin/%s...\n", target)
 	if err := e.git.Push("origin", target, false); err != nil {
 		return ProcessResult{