Add Spec Refine community extension to catalog and README (#2118)

* Add Spec Refine community extension to catalog and README

Adds the spec-kit-refine extension (4 commands, 2 hooks) that enables
iterative specification refinement — update specs in-place, propagate
changes to plan and tasks, diff impact, and track sync status.

Addresses community request in issue #1191 (101+ upvotes).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Fix alphabetical ordering of S-entries in Community Extensions table

Reorders Ship Release, Spec Critique, Spec Refine, Spec Sync, Staff Review,
and Superpowers Bridge into correct alphabetical order per publishing guide.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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