feat: improve catalog submission templates and CODEOWNERS (#2401)

Simplify the community catalog submission flow to use issue templates
with manual maintainer review (no automation scripts or workflows).

- Add explicit CODEOWNERS entries for catalog.community.json files so
  submissions are automatically assigned to a maintainer for review
- Improve preset submission template:
  - Add 'Required Extensions' optional field
  - Make 'Templates Provided' optional (supports command-only presets)
  - Add 'Number of Scripts' optional field

The existing extension and preset issue templates already collect all
required catalog metadata. Maintainers review submissions and manually
update the catalog JSON files.

Closes #2400