Add spec-kit-linear extension to community catalog (#2795)

* Add spec-kit-linear extension to community catalog

Add linear extension submitted by @ashbrener to:\n- extensions/catalog.community.json\n- docs/community/extensions.md\n\nCloses #2778

* Address PR review feedback for spec-kit-linear entry

- Use Unicode arrow (→) in catalog/docs description\n- Move docs row to alphabetical Spec section

* Address follow-up review naming/order feedback

- Use human-friendly display name: Linear Integration\n- Move docs row to alphabetical L section