Add Data Model Diagram extension to community catalog

Add data-model-diagram extension submitted by @benizzio to:
- extensions/catalog.community.json (alphabetical order)
- docs/community/extensions.md community extensions table

Closes #2920

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mnriem

docs/community/extensions.md

@@ -47,6 +47,7 @@ The following community-contributed extensions are available in [`catalog.commun
 | Conduct Extension | Orchestrates spec-kit phases via sub-agent delegation to reduce context pollution. | `process` | Read+Write | [spec-kit-conduct-ext](https://github.com/twbrandon7/spec-kit-conduct-ext) |
 | Confluence Extension | Create a doc in Confluence summarizing the specifications and planning files | `integration` | Read+Write | [spec-kit-confluence](https://github.com/aaronrsun/spec-kit-confluence) |
 | Cost Tracker | Track real LLM dollar cost across SDD workflows — per-feature budgets, per-integration comparison, and finance-ready exports | `visibility` | Read+Write | [spec-kit-cost](https://github.com/Quratulain-bilal/spec-kit-cost) |
+| Data Model Diagram | Generates Mermaid ER diagrams from Spec Kit data models after planning | `docs` | Read+Write | [spec-kit-data-model-diagram](https://github.com/benizzio/spec-kit-data-model-diagram) |
 | DocGuard — CDD Enforcement | Canonical-Driven Development enforcement. Validates, scores, and traces project documentation with automated checks, AI-driven workflows, and spec-kit hooks. One pinned runtime dependency; pure Node.js otherwise. | `docs` | Read+Write | [spec-kit-docguard](https://github.com/raccioly/docguard) |
 | Extensify | Create and validate extensions and extension catalogs | `process` | Read+Write | [extensify](https://github.com/mnriem/spec-kit-extensions/tree/main/extensify) |
 | Fix Findings | Automated analyze-fix-reanalyze loop that resolves spec findings until clean | `code` | Read+Write | [spec-kit-fix-findings](https://github.com/Quratulain-bilal/spec-kit-fix-findings) |

extensions/catalog.community.json

@@ -1,6 +1,6 @@
 {
   "schema_version": "1.0",
-  "updated_at": "2026-06-09T00:00:00Z",
+  "updated_at": "2026-06-10T00:00:00Z",
   "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.community.json",
   "extensions": {
     "aide": {
@@ -864,6 +864,38 @@
       "created_at": "2026-05-03T00:00:00Z",
       "updated_at": "2026-05-05T00:00:00Z"
     },
+    "data-model-diagram": {
+      "name": "Data Model Diagram",
+      "id": "data-model-diagram",
+      "description": "Generates Mermaid ER diagrams from Spec Kit data models after planning.",
+      "author": "benizzio",
+      "version": "0.2.2",
+      "download_url": "https://github.com/benizzio/spec-kit-data-model-diagram/archive/refs/tags/v0.2.2.zip",
+      "repository": "https://github.com/benizzio/spec-kit-data-model-diagram",
+      "homepage": "https://github.com/benizzio/spec-kit-data-model-diagram",
+      "documentation": "https://github.com/benizzio/spec-kit-data-model-diagram#readme",
+      "changelog": "https://github.com/benizzio/spec-kit-data-model-diagram/blob/main/CHANGELOG.md",
+      "license": "MIT",
+      "category": "docs",
+      "effect": "read-write",
+      "requires": {
+        "speckit_version": ">=0.2.0"
+      },
+      "provides": {
+        "commands": 1,
+        "hooks": 1
+      },
+      "tags": [
+        "diagram",
+        "documentation",
+        "mermaid"
+      ],
+      "verified": false,
+      "downloads": 0,
+      "stars": 0,
+      "created_at": "2026-06-10T00:00:00Z",
+      "updated_at": "2026-06-10T00:00:00Z"
+    },
     "diagram": {
       "name": "Spec Diagram",
       "id": "diagram",