docs: clarify setup, contributor checks, and troubleshooting

[RasputinKaiser]

Summary

• Add README navigation, requirements, first-use CLI examples, and source/cache/session version boundaries
• Correct the contributor guide's sync behavior against scripts/sync.sh, including rsync deletion and Claude marketplace caveats
• Add focused troubleshooting for discovery, validator finding codes, and skipped score-baseline comparisons

Scope

Documentation only: README.md, CONTRIBUTING.md, and docs/troubleshooting.md. Runtime code, manifests, dependencies, CI, license, security policy, releases, and git tags are unchanged.

Verification

Checked against source commit a88fad2; rechecked main before publication.

• Passed the complete current CI command set on a connector-materialized source snapshot with these documentation changes:
  • python3 scripts/validate.py: 11/11 checks
  • python3 skills/skill-curator/scripts/curator.py selftest: 87/87
  • python3 scripts/score.py selftest
  • python3 scripts/portfolio.py selftest
  • python3 scripts/calibrate.py selftest
  • python3 scripts/score.py . --min 70: effective score 74.4, Solid
• Ran README CLI examples against this repository as the explicit target: validator JSON, score JSON, and token report
• Checked 21 relative Markdown links/anchors in changed documents against the source tree and updated headings
• Reviewed the documentation diff and checked trailing whitespace
• Not run: actual Codex/Claude installation or sync.sh; live host compatibility remains unverified in this maintenance pass

No merge or release is requested.