Add new frontmatter fields from Claude Code 2.1.0

Update documentation and apply new frontmatter capabilities:

Documentation:
- Add user-invocable, context, agent, hooks fields to writing-skills.md
- Add disallowedTools, permissionMode, hooks fields to writing-agents.md
- Add model, context, hooks, allowed-tools fields to writing-commands.md
- Document skill hot-reload, built-in agents, background execution

Skills:
- Add user-invocable: false to gitea (CLI reference)
- Add user-invocable: false to repo-conventions (standards reference)

Commands:
- Add context: fork to heavy exploration commands (improve, plan-issues,
  create-repo, update-claude-md)
- Add missing argument-hint to roadmap, manifesto, improve

Agents:
- Add disallowedTools: [Edit, Write] to code-reviewer for safety

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/code-reviewer/AGENT.md

@@ -5,6 +5,9 @@ description: Automated code review of pull requests. Reviews PRs for quality, bu
 # The structured output format doesn't require opus-level reasoning.
 model: sonnet
 skills: gitea, code-review
+disallowedTools:
+  - Edit
+  - Write
 ---
 
 You are a code review specialist that provides immediate, structured feedback on pull request changes.