feat: Add Dart/Flutter-native install path for Stac skills

[divyanshub024]

Problem

Stac skills currently document installation through npx:

npx skills add https://github.com/StacDev/stac

That adds a Node/npm requirement for Flutter-first users who may already have Dart and Flutter installed, but not npx.

Proposal

Add a Stac-native install command, for example:

stac skills add

Optionally also support:

stac skills add https://github.com/StacDev/stac

This should install the skills from skills/catalog.json without requiring Node/npm.

Also add skills installation as an optional step in stac init, so new users can set up Stac and its agent skills in one flow.

Acceptance Criteria

• Users can install Stac skills without npx, Node, or npm.
• stac skills add installs all Stac skills by default.
• stac init optionally prompts users to install Stac skills.
• The installer reads from skills/catalog.json.
• docs/skills.mdx shows the Stac/Dart-native path first.
• The existing npx skills add https://github.com/StacDev/stac path can remain as an alternative.

Notes

• dart pub add may not be the right final UX because it is for adding package dependencies to pubspec.yaml.
• stac skills add likely fits better with the existing Stac CLI.
• stac init already follows Flutter-first tooling by trying flutter pub add stac and falling back to dart pub add stac.

[Pratikdate]

Hi! I'd like to work on this feature. Could you please assign it to me?

[divyanshub024]

Thanks @Pratikdate, I've assigned it to you.