Add V-Model Extension Pack to catalog (#1640)

* Add V-Model Extension Pack to catalog

Second community extension: V-Model paired dev-spec and test-spec
generation with regulatory-grade traceability.

- 3 commands: requirements, acceptance, trace
- Deterministic coverage validation (regex-based, not AI)
- Targets: IEC 62304, ISO 26262, DO-178C, FDA 21 CFR Part 820
- Repository: https://github.com/leocamello/spec-kit-v-model
- Release: v0.1.0

* Fix catalog entry: provides as number, add timestamps and statistics

Address review feedback:
- provides.commands: array → number (3), add hooks (1)
- Add created_at and updated_at timestamps
- Add statistics block (downloads: 0, stars: 0)

* Address review: use catalog.community.json and add extensions README

Per maintainer feedback:
- Revert catalog.json to its original empty state (empty by design)
- Rename catalog.example.json → catalog.community.json
- Replace example entries with real V-Model Extension Pack entry
- Add extensions/README.md with community extensions table