feat: add doc-guardian plugin for documentation lifecycle management

Adds automatic documentation drift detection and synchronization:
- PostToolUse hook detects when code changes affect docs
- Stop hook reminds of pending updates before session ends
- /doc-audit command for full project documentation scan
- /doc-sync command to batch apply pending updates
- doc-analyzer agent for cross-reference analysis
- doc-patterns skill for documentation structure knowledge

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

plugins/doc-guardian/skills/doc-patterns/SKILL.md

@@ -0,0 +1,39 @@
+---
+description: Knowledge of documentation patterns and structures for drift detection
+---
+
+# Documentation Patterns Skill
+
+## Common Documentation Structures
+
+### README.md Patterns
+- Installation section: version requirements, dependencies
+- Usage section: function calls, CLI commands
+- Configuration section: env vars, config files
+- API section: endpoint references
+
+### CLAUDE.md Patterns
+- Project context: tech stack versions
+- File structure: directory layout
+- Commands: available operations
+- Workflows: process descriptions
+
+### Code Documentation
+- Docstrings: function signatures, parameters, returns
+- Type hints: should match docstring types
+- Comments: inline references to other code
+
+## Drift Detection Rules
+
+1. **Version Mismatch**: Any hardcoded version in docs must match package.json, pyproject.toml, requirements.txt
+2. **Function References**: Function names in docs must exist in codebase with matching signatures
+3. **Path References**: File paths in docs must exist in current directory structure
+4. **Config Keys**: Environment variables and config keys in docs must be used in code
+5. **Command Examples**: CLI examples in docs should be valid commands
+
+## Priority Levels
+
+- **P0 (Critical)**: Broken references that would cause user errors
+- **P1 (High)**: Outdated information that misleads users
+- **P2 (Medium)**: Missing documentation for public interfaces
+- **P3 (Low)**: Style inconsistencies, minor wording issues