feat(fs): add FsStore var/tag test coverage and share validateName
Phase 4 of unified Store refactor (#38):
- Add FsVarStore tests (12 cases) and FsTagStore tests (11 cases)
covering CRUD, persistence-across-reopen, JSONL replay fidelity,
ListOptions, and error paths.
- Extract validateName into packages/core/src/validation.ts; remove
duplicated copies in core/src/store.ts and fs/src/var-store.ts.
- Fix FsTagStore.listByTag to honor ListOptions (limit/offset/desc)
via applyListOptions, matching the in-memory implementation.
- Replace stale openStoreAndVarStore example in usage.md with
openStore returning OcasStore; add grep-based regression test.
- Add OcasStore shape assertion in fs/src/store.test.ts.
Closes #42; partially addresses #47 (items 1, 3).
xiaoju
9965e75c22