feat(catalog): add Cost Tracker (cost) community extension (#2448)

* feat(catalog): add Cost Tracker (cost) community extension

Adds a new entry for spec-kit-cost — track real LLM dollar cost across
SDD workflows with per-feature budgets, per-integration comparison,
and finance-ready exports.

Repo: https://github.com/Quratulain-bilal/spec-kit-cost
Release: v1.0.0

* docs(catalog): add Cost Tracker README row, bump updated_at

Address Copilot review feedback:
- Add Cost Tracker row to README community extensions table
- Bump top-level updated_at per EXTENSION-PUBLISHING-GUIDE.md

* fix(catalog): address Copilot feedback on cost extension entry

- Move cost entry after confluence so the c* block is alphabetized
- Bump top-level updated_at to 2026-05-05 per EXTENSION-PUBLISHING-GUIDE
- Use documented 'visibility' category in README (not 'analytics'),
  matching Token Consumption Analyzer's classification
- Replace 'analytics' tag with 'visibility' in catalog tags for consistency

* fix(catalog): bump top-level updated_at for cost entry addition

Address Copilot feedback: the file-level updated_at must be bumped on
every catalog change per EXTENSION-PUBLISHING-GUIDE.md:204-205.

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Co-authored-by: Quratulain-bilal <quratulain.bilal@users.noreply.github.com>