feat: add persistent artifact storage for team sharing

Add .codebase-memory/graph.db.zst — a zstd-compressed knowledge graph
artifact that can be committed to the repo. Teammates bootstrap from
the artifact instead of running a full reindex from scratch.

- Vendor zstd 1.5.7 (amalgamated build) for 8-13:1 compression
- Two-tier export: zstd -9 + index stripping for explicit index,
  zstd -3 for watcher/incremental auto-updates
- Import: decompress → integrity check → auto-recreate indexes
- Bootstrap in handle_index_repository: when no local DB exists but
  artifact is present, import first then run incremental
- Auto-create .gitattributes with merge=ours to prevent conflicts
- Fix: add missing idx_edges_url_path to create_user_indexes and
  url_path_gen generated column to init_schema
- 13 new tests (5 zstd wrapper + 8 artifact round-trip/edge cases)