Update agents to use YAML frontmatter format

- Update product-manager agent to use official Claude Code format
- Update ARCHITECTURE.md to document frontmatter fields
- Add code-reviewer agent and code-review skill to docs
- Update README.md project structure

This aligns our agent documentation with the official Claude Code
agent format which uses YAML frontmatter for name, description,
model, skills, and tools.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -100,9 +100,11 @@ ai/
 │   ├── forgejo/        # Forgejo CLI integration
 │   ├── issue-writing/  # Issue structure best practices
 │   ├── backlog-grooming/  # Backlog maintenance
-│   └── roadmap-planning/  # Feature breakdown
+│   ├── roadmap-planning/  # Feature breakdown
+│   └── code-review/    # Code review best practices
 ├── agents/             # Specialized subagents
-│   └── product-manager/  # Combines skills for PM tasks
+│   ├── product-manager/  # Combines skills for PM tasks
+│   └── code-reviewer/  # Automated PR code review
 ├── scripts/            # Git hooks and utilities
 │   └── pre-commit-checks.sh
 ├── settings.json       # Claude Code configuration