Add Spec Kit Discovery Extension to community catalog

[bigsmartben]

Description

Adds the Spec Kit Discovery Extension to the community extension catalog using the community submission flow.

This PR updates only the two maintainer-owned catalog surfaces required by the community extension workflow:

• extensions/catalog.community.json
• docs/community/extensions.md

The submitted extension version is 0.2.0, with 6 commands and no hooks. The existing upstream submission issue was updated to match v0.2.0: github#3113.

Note: I could not add the extension-submission label to the upstream issue because this account does not have label permissions on github/spec-kit.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Additional validation performed:

• python -c "import json; json.load(open('extensions/catalog.community.json', encoding='utf-8')); print('Valid JSON')"
• catalog ID ordering check for extensions/catalog.community.json
• verified v0.2.0 release exists and is not draft/prerelease
• curl.exe -I -L https://github.com/bigsmartben/spec-kit-discovery/archive/refs/tags/v0.2.0.zip returned a final 200 OK zip response
• installed from the release URL into a temporary Spec Kit project with explicit trust confirmation
• confirmed specify extension list reports Commands: 6 | Hooks: 0
• PYTHONPATH=src uv run pytest tests/test_extensions.py::TestExtensionCatalog -q -> 54 passed
• git diff --check

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

OpenAI Codex helped inspect the community extension submission rules, update the issue body to v0.2.0, add the catalog/docs entries, run validation checks, and draft this PR body.

References github#3113.