Add Spec Diagram community extension to catalog and README (#2129)

Adds the spec-kit-diagram extension (3 commands, 1 hook) that auto-generates
Mermaid diagrams for SDD workflow visualization, feature progress tracking,
and task dependency graphs.

Addresses community request in issue #467 (50+ upvotes).

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Quratulain-bilal

README.md

@@ -224,6 +224,7 @@ The following community-contributed extensions are available in [`catalog.commun
 | Security Review | Comprehensive security audit of codebases using AI-powered DevSecOps analysis | `code` | Read-only | [spec-kit-security-review](https://github.com/DyanGalih/spec-kit-security-review) |
 | Ship Release Extension | Automates release pipeline: pre-flight checks, branch sync, changelog generation, CI verification, and PR creation | `process` | Read+Write | [spec-kit-ship](https://github.com/arunt14/spec-kit-ship) |
 | Spec Critique Extension | Dual-lens critical review of spec and plan from product strategy and engineering risk perspectives | `docs` | Read-only | [spec-kit-critique](https://github.com/arunt14/spec-kit-critique) |
+| Spec Diagram | Auto-generate Mermaid diagrams of SDD workflow state, feature progress, and task dependencies | `visibility` | Read-only | [spec-kit-diagram-](https://github.com/Quratulain-bilal/spec-kit-diagram-) |
 | Spec Refine | Update specs in-place, propagate changes to plan and tasks, and diff impact across artifacts | `process` | Read+Write | [spec-kit-refine](https://github.com/Quratulain-bilal/spec-kit-refine) |
 | Spec Sync | Detect and resolve drift between specs and implementation. AI-assisted resolution with human approval | `docs` | Read+Write | [spec-kit-sync](https://github.com/bgervin/spec-kit-sync) |
 | Staff Review Extension | Staff-engineer-level code review that validates implementation against spec, checks security, performance, and test coverage | `code` | Read-only | [spec-kit-staff-review](https://github.com/arunt14/spec-kit-staff-review) |

extensions/catalog.community.json

@@ -322,6 +322,38 @@
       "created_at": "2026-03-29T00:00:00Z",
       "updated_at": "2026-03-29T00:00:00Z"
     },
+    "diagram": {
+      "name": "Spec Diagram",
+      "id": "diagram",
+      "description": "Auto-generate Mermaid diagrams of SDD workflow state, feature progress, and task dependencies.",
+      "author": "Quratulain-bilal",
+      "version": "1.0.0",
+      "download_url": "https://github.com/Quratulain-bilal/spec-kit-diagram-/archive/refs/tags/v1.0.0.zip",
+      "repository": "https://github.com/Quratulain-bilal/spec-kit-diagram-",
+      "homepage": "https://github.com/Quratulain-bilal/spec-kit-diagram-",
+      "documentation": "https://github.com/Quratulain-bilal/spec-kit-diagram-/blob/main/README.md",
+      "changelog": "https://github.com/Quratulain-bilal/spec-kit-diagram-/blob/main/CHANGELOG.md",
+      "license": "MIT",
+      "requires": {
+        "speckit_version": ">=0.4.0"
+      },
+      "provides": {
+        "commands": 3,
+        "hooks": 1
+      },
+      "tags": [
+        "diagram",
+        "mermaid",
+        "visualization",
+        "workflow",
+        "dependencies"
+      ],
+      "verified": false,
+      "downloads": 0,
+      "stars": 0,
+      "created_at": "2026-04-08T00:00:00Z",
+      "updated_at": "2026-04-08T00:00:00Z"
+    },
     "docguard": {
       "name": "DocGuard — CDD Enforcement",
       "id": "docguard",