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/health-tracker/README.md

@@ -0,0 +1,23 @@
+# Health Tracker
+
+Wellness tracking agent for health metrics, medication reminders, fitness goals, and lifestyle habits.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Module | `builtin:chat` |
+| Model | `default` |
+| Provider | `default` |
+
+System prompt instructs health metrics logging, medication management, fitness goal tracking, habit building, and wellness reporting. Includes medical disclaimer.
+
+## Skills
+
+- `file_read`, `file_write`, `file_list`, `memory_store`, `memory_recall`
+
+## Usage
+
+```bash
+librefang agent run health-tracker
+```