Add TweetClaw OpenClaw preflight case study

[kriptoburak]

Summary

• Add a case study for using agentsec as a preflight scan on the TweetClaw OpenClaw plugin.
• Include a sanitized scan summary artifact with 0 findings from the skill scanner.
• Link the new case study from the README case-study list.

Changes

• Documents the review steps for a high-capability X/Twitter automation plugin.
• Uses npm as the canonical package source with @xquik/tweetclaw.
• Calls out keeping the Xquik API key outside the repository, package, and scan artifacts.

Testing Performed

• agentsec scan /Users/burak/Developer/tweetclaw -s skill -o json -f /tmp/tweetclaw-agentsec-skill-scan-repeat.json --fail-on critical passes.
• git diff --cached --check passes.
• jq empty docs/case-studies/artifacts/case5-tweetclaw-skill-scan.json passes.
• Checked the added external URL and local README link target.
• ruff check src/ tests/ passes. Not run, docs-only change.
• ruff format --check src/ tests/ passes. Not run, docs-only change.
• mypy src/agentsec/ --ignore-missing-imports passes. Not run, docs-only change.
• pytest tests/ -v --tb=short passes. Not run, docs-only change.

Risk & Rollback

• Risks: low, documentation and sanitized JSON artifact only.
• Rollback plan: revert this commit to remove the case study and README link.