Add Improve Extension to community catalog

[mnriem]

Adds the Improve Extension (improve) submitted by @d0whc3r to the community catalog.

Closes #2972

Validation summary

Check	Result
Repository exists and is public	✅ https://github.com/d0whc3r/spec-kit-improve
extension.yml manifest present	✅
README.md present	✅
LICENSE file present	✅ MIT
GitHub release matching version	✅ v1.0.1
Download URL accessible	✅ HTTP 200
Extension ID lowercase-with-hyphens	✅ improve
Version follows semver	✅ 1.0.1
extension.yml agrees with submission	✅ version, command count (1), hook count (0), tags

Changes

• extensions/catalog.community.json — new improve entry in alphabetical order; top-level updated_at refreshed
• docs/community/extensions.md — new row in the community extensions table (alphabetical by name)

cc @d0whc3r

[Copilot]

Pull request overview

Adds the “Improve Extension” (improve) to the community extensions catalog so it can be discovered via Spec Kit’s community reference catalog and listed in the docs table.

Changes:

• Added a new improve entry to extensions/catalog.community.json (sorted by extension id) and refreshed the catalog-level updated_at.
• Added “Improve Extension” to the community extensions table in docs/community/extensions.md (sorted by extension name).

Show a summary per file

File	Description
extensions/catalog.community.json	Adds the improve catalog entry (with category/effect, requires/provides, tags, timestamps) and updates the top-level updated_at.
docs/community/extensions.md	Adds a corresponding table row for “Improve Extension” in alphabetical order.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 0