feat: detect uncommitted JSONL changes before sync (GH#885, bd-vd8e)

Add pre-flight safety check to detect when a previous sync exported
but failed before commit, leaving JSONL in an inconsistent state.

- Add gitHasUncommittedBeadsChanges() helper in sync_git.go
- Call in sync pre-flight checks after merge/rebase check
- If uncommitted changes detected, force re-export to reconcile state

This catches the failure mode early before it compounds across worktrees.

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

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

cmd/bd/sync.go

@@ -163,6 +163,21 @@ Use --merge to merge the sync branch back to main branch.`,
 			FatalErrorWithHint("unmerged paths or merge in progress", "resolve conflicts, run 'bd import' if needed, then 'bd sync' again")
 		}
 
+		// GH#885: Preflight check for uncommitted JSONL changes
+		// This detects when a previous sync exported but failed before commit,
+		// leaving the JSONL in an inconsistent state across worktrees.
+		if hasUncommitted, err := gitHasUncommittedBeadsChanges(ctx); err != nil {
+			fmt.Fprintf(os.Stderr, "Warning: failed to check for uncommitted changes: %v\n", err)
+		} else if hasUncommitted {
+			fmt.Println("→ Detected uncommitted JSONL changes (possible incomplete sync)")
+			fmt.Println("→ Re-exporting from database to reconcile state...")
+			// Force a fresh export to ensure JSONL matches current DB state
+			if err := exportToJSONL(ctx, jsonlPath); err != nil {
+				FatalError("re-exporting to reconcile state: %v", err)
+			}
+			fmt.Println("✓ State reconciled")
+		}
+
 		// GH#638: Check sync.branch BEFORE upstream check
 		// When sync.branch is configured, we should use worktree-based sync even if
 		// the current branch has no upstream (e.g., detached HEAD in jj, git worktrees)