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.

skills/custom-skill-python/README.md

@@ -0,0 +1,20 @@
+# word-counter
+
+Count words, sentences, and characters in text. A minimal example of a Python runtime skill.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Type | `python` |
+| Entry | `main.py` |
+
+## Input
+
+- **text** (string, required) — The text to analyze
+
+## Usage
+
+```bash
+librefang skill test ./skills/custom-skill-python --input '{"text": "Hello world. How are you?"}'
+```