test: validate skills map entries and add contributing guide

[tonyblu331]

What Changed

• New: packages/autoskills/tests/skills-map.validation.test.mjs — 12 structural validation tests across 3 suites
• New: CONTRIBUTING.md — AutoSkills-specific contribution guide

Validation test — what it checks

SKILLS_MAP:

• Unique technology ids
• Required fields present (id, name, detect, skills)
• detect has at least one signal (packages, configFiles, gems, packagePatterns, or configFileContent)
• Signal arrays are non-empty with correct types (RegExp for packagePatterns, strings for the rest)
• configFileContent blocks have patterns and either files or scanGradleLayout: true
• Skill refs match owner/repo/skill-name or https://...
• No duplicate skill refs within an entry

COMBO_SKILLS_MAP:

• Unique combo ids
• requires has ≥ 2 ids, all of which exist in SKILLS_MAP
• No duplicate ids within requires
• Same skill ref validation as above

Error feedback contract (8 additional tests): validates that error messages are actionable — they tell the contributor exactly what to fix and include format hints or examples.

Why This Change

Closes #62.

Discussion in #62 concluded that the registry split proposal was premature at the current scale (~1,300 lines of declarative data). The pragmatic path: add a schema validation test and a clear contribution guide without changing the architecture or raising the bar for contributors.

This PR delivers exactly that:

• Single-map authoring workflow stays intact
• No build step introduced
• CI now rejects malformed entries with specific, actionable messages

Testing Done

• ✅ Automated tests pass locally: 171 tests, 0 failures
• ✅ Edge cases tested: configFileContent variants, combo references, duplicate detection, type mismatches
• ✅ Error messages are actionable with format hints

Type of Change

• test: Tests
• docs: Documentation

Security & Quality Checklist

• No secrets or API keys committed
• Follows project coding standards (ESM, node:test, node:assert/strict)
• No sensitive data exposed

Documentation

• CONTRIBUTING.md added with full workflow + AutoSkills-specific guidance
• Detection signal priority table with code examples
• Common mistakes shown with ✗ / ✓ annotations
• Exact error output contributors will see in CI
• Separate checklists for technology, combo, and detection fix PRs

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