chore(skills): publish Remix skills on skills.sh

[leoafarias]

Description

Makes skills/ the repository's only committed skill catalog and prepares its two public skills for distribution from GitHub through skills.sh:

• Keeps using-remix and building-remix-design-system, with standard agents/openai.yaml metadata for both.
• Consolidates Fortal into using-remix as a deferred reference, reduces the main skill to package/setup/styling/host routing, splits component guidance by category, and expands Fortal-specific evaluation coverage.
• Removes the root .agents mirrors and .claude/skills symlink, while ignoring future local installations.
• Documents the skills.sh pages, badge, and install/list/use/update workflows, including global and ephemeral usage for contributors who do not want a local .agents directory.

No Dart or Flutter public APIs change.

Validation

• Skill validation script for both public skill directories — passed
• Clean-room npx skills add copies for both skills — passed; all referenced files were included
• DISABLE_TELEMETRY=1 npx --yes skills add . --list — passed; reported exactly two skills
• fvm dart run melos run docs:check — passed
• fvm dart run melos run fortal:parity:check — passed
• git diff --check — passed

Related Issues

None.

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Checklist

Note: Updating the pubspec.yaml and CHANGELOG.md is not required. These are handled automatically during the release process.

• My PR includes unit or integration tests for all changed/updated/fixed behaviors.
• I have updated or added relevant documentation (doc comments with ///).
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.