docs: clarify bookmark review safety boundaries

[kriptoburak]

Summary

• add a first-run approval checklist and explicit read-only boundaries
• document TweetClaw as an optional, separately approved Xquik evidence companion
• add the missing Skill metadata required by agent skill validators
• repair root-relative example links so they work on GitHub and installed copies
• clarify configured external services and local artifact handling

Independent Repository Fix

SKILL.md did not have YAML frontmatter, so standard agent skill validation failed with No YAML frontmatter found. This change adds the required name and trigger-focused description metadata. It also repairs the README example links, which previously resolved from the repository root instead of the README location.

Validation

• quick_validate.py .
• markdown-link-check README.md SKILL.md examples/example-analysis.md
• jq empty examples/example-artifact.json
• git diff --check

The Xquik reference is limited to an optional companion workflow. Bookmark-brain remains responsible only for bookmark review and analysis.