DRY install docs, centralize skill registry, add validation

[FineComputer14451]

Summary

Addresses maintainability findings from the code review: duplicated install commands and skill tables across README, AGENTS.md, and the workflow setup guide.

• docs/install-skills.md — single source for bash, PowerShell, script, and manual install
• docs/skills-registry.md — canonical 13-skill table with prompts
• scripts/install-skills.sh / install-skills.ps1 — copy-pasteable installers
• scripts/validate-skills.sh — checks frontmatter, ## Grok Build Mode, and registry coverage
• README trimmed from 326 → 273 lines; comparison section links to docs/comparison.md

Verification

scripts/validate-skills.sh
# validate-skills: all checks passed

CI follow-up

A local commit adds a validate-skills job to .github/workflows/markdown-check.yml, but it could not be pushed because the current gh token lacks the workflow scope. After merging, either:

1. Run gh auth refresh -h github.com -s workflow and push the workflow change, or
2. Add this job manually:

  validate-skills:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: scripts/validate-skills.sh

Risks

Low — documentation and tooling only; no skill behavior changes.

Follow-ups

• Extract shared ## Grok Build Mode boilerplate into a referenced fragment
• Normalize skill versions to a documented policy

Sources

Community-maintained refactor; no official xAI claims changed.