docs(tools): add coherence tools README

tools/coherence/README.md

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+# Coherence Tools
+
+Tools for maintaining site coherence.
+
+## Tools
+
+| Tool | Purpose | Usage |
+|------|---------|-------|
+| check-frontmatter.py | Validates YAML frontmatter exists | `python3 check-frontmatter.py <dir>` |
+
+## Usage
+
+```bash
+# Check all fieldnotes
+python3 check-frontmatter.py ../public/fieldnotes/
+
+# Check single file
+python3 check-frontmatter.py file.md
+```
+
+## Exit Codes
+
+- 0: All files have frontmatter
+- 1: Some files missing frontmatter
+- 2: Invalid arguments
+
+## Adding New Tools
+
+1. Create new tool in this directory
+2. Add entry to this README
+3. Document usage and exit codes