Remove related_skills from SKILL.md frontmatter (dead metadata)

what:
- Remove 'related_skills' line from SKILL.md YAML frontmatter
- Thoroughly review every file in the repo for stale references

why:
related_skills is agent-specific frontmatter not read by any major runtime
(Hermes, Claude Code, Codex, Cursor). We already concluded in the
investigation that extra frontmatter fields are dead bytes. Removing it
eliminates speculative metadata with zero observed value.

Thorough review confirmed zero stale references remain:
- No python-best-practices-skill references (old repo name)
- No review-checklist references (deleted file)
- No run_phase2_checks or run_benchmark references (deleted scripts)
- 13 tracked files, all consistent with current scope

Author: CodeSigils

skill/SKILL.md

@@ -16,7 +16,6 @@ ref:
 metadata:
   hermes:
     tags: [python, project-setup, workflow, tooling, verification]
-    related_skills: [codebase-inspection, shell-scripting]
 ---
 
 # Python Project Workflow