docs: add README for every content directory

Each directory (agents, hands, integrations, plugins, providers,
scripts, skills) now has a README documenting its TOML format,
current contents, and contribution steps.

scripts/README.md

@@ -0,0 +1,35 @@
+# Scripts
+
+Utility scripts for maintaining the LibreFang registry.
+
+## validate.py
+
+Validates all TOML content files across the registry.
+
+```bash
+python scripts/validate.py
+```
+
+### What It Checks
+
+**Per content type:**
+- **Providers** -- required fields, valid tiers, non-negative costs, no duplicate model IDs
+- **Agents** -- required fields (name, description, module), name matches directory
+- **Hands** -- required fields (id, name, description), valid category, [agent] section, id matches directory
+- **Integrations** -- required fields (id, name), [transport] section, id matches filename
+- **Skills** -- [skill] section with name, [runtime] with valid type
+- **Plugins** -- name matches directory, [hooks] section, hook files exist
+
+**Cross-type checks:**
+- Routing alias collisions between agents and hands (reported as warnings)
+- Cross-file duplicate model IDs within the same provider
+
+### Requirements
+
+- Python 3.11+ (uses `tomllib`)
+- Or Python 3.8+ with `pip install tomli`
+
+### Exit Codes
+
+- `0` -- all checks passed
+- `1` -- one or more validation errors