Add Spec Kit Discovery Extension to community catalog (#3119)

Add discovery extension submitted by @bigsmartben to:
- extensions/catalog.community.json (alphabetical order)
- docs/community/extensions.md community extensions table

Closes #3113

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/catalog.community.json

@@ -1001,6 +1001,40 @@
       "created_at": "2026-04-08T00:00:00Z",
       "updated_at": "2026-04-08T00:00:00Z"
     },
+    "discovery": {
+      "name": "Spec Kit Discovery Extension",
+      "id": "discovery",
+      "description": "Run technical discovery commands for feasibility, technology selection, scenario-specific technical decisions, legacy codebase assessment, implementation understanding, and proof-of-concept validation.",
+      "author": "bigsmartben",
+      "version": "0.2.0",
+      "download_url": "https://github.com/bigsmartben/spec-kit-discovery/archive/refs/tags/v0.2.0.zip",
+      "repository": "https://github.com/bigsmartben/spec-kit-discovery",
+      "homepage": "https://github.com/bigsmartben/spec-kit-discovery",
+      "documentation": "https://github.com/bigsmartben/spec-kit-discovery/blob/main/docs/usage.md",
+      "changelog": "https://github.com/bigsmartben/spec-kit-discovery/blob/main/CHANGELOG.md",
+      "license": "MIT",
+      "category": "process",
+      "effect": "read-write",
+      "requires": {
+        "speckit_version": ">=0.1.0"
+      },
+      "provides": {
+        "commands": 6,
+        "hooks": 0
+      },
+      "tags": [
+        "discovery",
+        "workflow",
+        "validation",
+        "feasibility",
+        "decision"
+      ],
+      "verified": false,
+      "downloads": 0,
+      "stars": 0,
+      "created_at": "2026-06-23T00:00:00Z",
+      "updated_at": "2026-06-23T00:00:00Z"
+    },
     "docguard": {
       "name": "DocGuard — CDD Enforcement",
       "id": "docguard",