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/catalog.community.json

@@ -0,0 +1,33 @@
+{
+  "schema_version": "1.0",
+  "updated_at": "2026-02-20T00:00:00Z",
+  "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.community.json",
+  "extensions": {
+    "v-model": {
+      "name": "V-Model Extension Pack",
+      "id": "v-model",
+      "description": "Enforces V-Model paired generation of development specs and test specs with full traceability.",
+      "author": "leocamello",
+      "version": "0.1.0",
+      "download_url": "https://github.com/leocamello/spec-kit-v-model/archive/refs/tags/v0.1.0.zip",
+      "repository": "https://github.com/leocamello/spec-kit-v-model",
+      "homepage": "https://github.com/leocamello/spec-kit-v-model",
+      "documentation": "https://github.com/leocamello/spec-kit-v-model/blob/main/README.md",
+      "changelog": "https://github.com/leocamello/spec-kit-v-model/blob/main/CHANGELOG.md",
+      "license": "MIT",
+      "requires": {
+        "speckit_version": ">=0.1.0"
+      },
+      "provides": {
+        "commands": 3,
+        "hooks": 1
+      },
+      "tags": ["v-model", "traceability", "testing", "compliance", "safety-critical"],
+      "verified": false,
+      "downloads": 0,
+      "stars": 0,
+      "created_at": "2026-02-20T00:00:00Z",
+      "updated_at": "2026-02-20T00:00:00Z"
+    }
+  }
+}