docs(presets): add lean preset README and enrich catalog metadata (#2340)

* docs(presets): add lean preset README and enrich catalog metadata

- Add README.md documenting the lean workflow preset, its commands,
  when to use it, and development instructions.
- Add license, requires.speckit_version, and provides.commands fields
  to the lean preset catalog entry.
- Add "core" tag to preset.yml for discoverability.

* fix: bump catalog updated_at and add provides.templates for consistency

Address PR review feedback:
- Bump updated_at to reflect catalog modification time
- Add provides.templates (0) to lean preset entry for consistency
  with catalog schema used in catalog.community.json