feat(skills): add explicit skill sources - #49114

Issue for this PR

Closes #49108

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenCode discovers skills from several compatibility directories. Separate Claude, Agent, OpenCode, model-version, and A/B skill trees can intentionally contain the same skill directory names, but the current loader stores them under one name-keyed collection and loads them concurrently. This makes collisions ambiguous and can fail while constructing the system prompt.

This PR adds explicit skill source configuration:

{
  "skills": {
    "sources": [
      { "id": "opencode", "path": "~/.config/opencode/skills", "enabled": true },
      { "id": "agents", "path": "~/.agents/skills", "enabled": true },
      { "id": "claude", "path": "~/.claude/skills", "enabled": true }
    ],
    "collision": "source-qualified"
  }
}

Explicit sources replace implicit compatibility-root discovery. In source-qualified mode, same-named skills remain separate as claude/review and agents/review. In last-wins mode, duplicate selection is deterministic because discovered files load in stable order. Existing discovery behavior remains unchanged when explicit sources are absent. Malformed skill files continue through the existing diagnostic path without crashing.

The implementation is limited to configuration schema, skill discovery, deterministic loading, and regression coverage. It does not replace or duplicate the compatibility-loader refactor in #48881; it adds the user-facing source-selection and collision policy on top of the loader contract.

How did you verify your code works?

• bun test test/skill/skill.test.ts: 19 passed, 0 failed, 53 assertions
• bun run typecheck: passed
• bunx prettier --check: passed
• bunx oxlint: 0 warnings, 0 errors
• Local fork runtime check: debug skill --pure reported both agents/review and claude/review from an isolated config
• Repository pre-push hook: 30 typecheck tasks passed

Screenshots / recordings

Not applicable; this is a configuration and backend behavior change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.