Fix bd-hv01: Implement deletion tracking for multi-workspace sync

- Add 3-way merge deletion tracking using snapshot files
- Create .beads/beads.base.jsonl and .beads/beads.left.jsonl snapshots
- Integrate into both sync.go and daemon_sync.go
- Add comprehensive test suite in deletion_tracking_test.go
- Update .gitignore to exclude snapshot files

This fixes the resurrection bug where deleted issues come back after
multi-workspace git sync. Uses the beads-merge 3-way merge logic to
detect and apply deletions correctly.

.gitignore

@@ -49,6 +49,10 @@ Thumbs.db
 # Keep JSONL exports (source of truth for git)
 !.beads/*.jsonl
 
+# 3-way merge snapshot files (local-only, for deletion tracking)
+.beads/beads.base.jsonl
+.beads/beads.left.jsonl
+
 # Ignore nix result
 result