feat: add custom agents and slash commands

Add comprehensive custom agents for specialized workflows:
- gentoo-sysadmin: Gentoo Linux system administration (OpenRC, Portage)
- tdd-test-writer: TDD Ping-Pong workflow implementation
- code-reviewer: Code review against strict style guidelines
- documentation-writer: Comprehensive project documentation
- python-ai-tutor: AI/ML coursework assistant with teaching focus
- dotfiles-manager: Dotfiles management with chezmoi

Add custom slash commands for quick workflows:
- /gentoo-update: Safe Gentoo system update procedure
- /tdd-next: Start next TDD test cycle
- /style-check: Comprehensive code style validation
- /doc-sync: Update all project documentation
- /review-pr: Review pull request before merge
- /script-install: Install system script with proper permissions

Updated .gitignore to properly track agents and commands while
excluding session-specific data (plans/, etc.)

Added README.md documenting the configuration structure and usage.

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

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

.gitignore

@@ -41,6 +41,9 @@ file-history/
 # Shell state snapshots (temporary shell state)
 shell-snapshots/
 
+# Plan mode drafts (session-specific planning)
+plans/
+
 # =============================================================================
 # LOGS AND DEBUG DATA
 # =============================================================================
@@ -129,14 +132,19 @@ Desktop.ini
 # =============================================================================
 
 # These files SHOULD be tracked and are explicitly listed here for clarity:
-# !CLAUDE.md              - Your global Claude instructions
-# !settings.json          - Claude Code settings
-# !agents/                - Custom agents
-# !commands/              - Custom slash commands
-# !skills/                - Custom skills
-# !hooks/                 - Custom hooks
-# !.clauderc              - Claude configuration
-# !README.md              - Documentation for your setup
+# The following patterns ensure custom configurations are tracked in git:
+#
+# - CLAUDE.md              - Your global Claude instructions
+# - settings.json          - Claude Code settings
+# - agents/                - Custom agents (*.json files)
+# - commands/              - Custom slash commands (*.md files)
+# - skills/                - Custom skills
+# - hooks/                 - Custom hooks
+# - .clauderc              - Claude configuration
+# - README.md              - Documentation for your setup
+#
+# These are NOT ignored by default and will be tracked when you git add them.
+# The .gitignore above only excludes machine-specific and sensitive data.
 
 # =============================================================================
 # ADDITIONAL PATTERNS