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

extensions/README.md

@@ -0,0 +1,13 @@
+# Spec Kit Community Extensions
+
+Community-contributed extensions for [Spec Kit](https://github.com/github/spec-kit).
+
+## Available Extensions
+
+| Extension | Purpose | URL |
+|-----------|---------|-----|
+| V-Model Extension Pack | Enforces V-Model paired generation of development specs and test specs with full traceability | [spec-kit-v-model](https://github.com/leocamello/spec-kit-v-model) |
+
+## Adding Your Extension
+
+See the [Extension Publishing Guide](EXTENSION-PUBLISHING-GUIDE.md) for instructions on how to submit your extension to the community catalog.