docs(readme): list red-team in community-extensions table (#2311)

Follow-up to #2306 (merged). Per maintainer request
(https://github.com/github/spec-kit/pull/2306#issuecomment-4296655643),
adds the red-team entry to the alphabetically-ordered community-extensions
table in README.md so the extension is discoverable alongside the other
community entries — not only via catalog.community.json.

Slotted alphabetically between "Reconcile Extension" and "Repository
Index". Category: docs. Effect: Read+Write (produces a structured
findings-report file at specs/<feature-id>/red-team-findings-*.md; does
not modify specs — every resolution is maintainer-authorised).

Co-authored-by: Ash Brener <ashley@midletearth.com>