docs: add README for every subdirectory

33 agent READMEs, 14 hand READMEs, 2 plugin READMEs
(echo-memory + hooks), and 2 skill READMEs
(custom-skill-prompt + custom-skill-python).

Each README documents the component's purpose, configuration,
and usage based on its TOML definition.

agents/router/README.md

@@ -0,0 +1,23 @@
+# Router
+
+Native deterministic router. Dispatches tasks to specialist agents without using an LLM.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Module | `builtin:router` |
+| Model | `default` |
+| Provider | `default` |
+
+This is a system-level agent. The `builtin:router` module performs deterministic routing and does not invoke an LLM.
+
+## Skills
+
+None configured.
+
+## Usage
+
+```bash
+librefang agent run router
+```