feat(engineering): integrate focused-fix skill — docs, command, agent, marketplace

- Normalize SKILL.md frontmatter to repo standard (remove non-standard
  license, metadata.* fields; inline description)
- Generate docs page (docs/skills/engineering/focused-fix.md)
- Add to mkdocs.yml nav (skills + commands)
- Create /focused-fix slash command (commands/ + .claude/commands/)
- Add to cs-senior-engineer agent (skill integration + new workflow #4)
- Update marketplace.json and plugin.json descriptions (30 → 31 skills)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mkdocs.yml

@@ -181,6 +181,7 @@ nav:
       - "Database Schema Designer": skills/engineering/database-schema-designer.md
       - "Dependency Auditor": skills/engineering/dependency-auditor.md
       - "Env & Secrets Manager": skills/engineering/env-secrets-manager.md
+      - "Focused Fix": skills/engineering/focused-fix.md
       - "Git Worktree Manager": skills/engineering/git-worktree-manager.md
       - "Interview System Designer": skills/engineering/interview-system-designer.md
       - "MCP Server Builder": skills/engineering/mcp-server-builder.md
@@ -372,6 +373,7 @@ nav:
     - "/code-to-prd": commands/code-to-prd.md
     - "/competitive-matrix": commands/competitive-matrix.md
     - "/financial-health": commands/financial-health.md
+    - "/focused-fix": commands/focused-fix.md
     - "/okr": commands/okr.md
     - "/persona": commands/persona.md
     - "/pipeline": commands/pipeline.md