docs: use installed ghcrawl binary in skill docs

[huntharo]

Summary

• remove repo-local pnpm and npx fallback guidance from the shipped ghcrawl skill
• require the installed ghcrawl binary as the normal operator surface
• point maintainers who explicitly want source-tree commands to CONTRIBUTING.md

Verification

• python /Users/huntharo/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/ghcrawl (run in a temporary venv with PyYAML installed)

Closes #25

[github-actions]

Cluster Performance

• Status: PASS
• Fixture median: 491.4 ms (12 samples, 3 cluster rebuilds/sample)
• Fixture baseline: 535.1 ms
• Fixture delta: -43.7 ms (-8.2%)
• Projected openclaw/openclaw duration: 9m 11.0s
• Projected openclaw/openclaw baseline: 10m 0.0s
• Projected delta: -48996.6 ms (-8.2%)
• Regression threshold: +50.0%
• Fixture shape: 512 threads x 3 source kinds
• Sample durations: 515.9 ms, 493.5 ms, 513.0 ms, 487.3 ms, 517.2 ms, 502.9 ms, 510.9 ms, 475.1 ms, 472.7 ms, 489.3 ms, 482.3 ms, 473.9 ms

Run: workflow run for 00c61fd

[huntharo]

Hmm... this is not needed. Turns out we preferred the installed package if it was there.