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>

skills/gitea/SKILL.md

@@ -1,6 +1,7 @@
 ---
 name: gitea
 description: View, create, and manage Gitea issues and pull requests using tea CLI. Use when working with issues, PRs, viewing issue details, creating pull requests, adding comments, merging PRs, or when the user mentions tea, gitea, issue numbers, or PR numbers.
+user-invocable: false
 ---
 
 # Gitea CLI (tea)