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.

hands/apitester/README.md

@@ -0,0 +1,32 @@
+# API Tester Hand
+
+Autonomous API testing agent -- endpoint discovery, request validation, load testing, and regression detection.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Category | `development` |
+| Agent | `apitester-hand` |
+| Routing | `api test`, `endpoint test`, `load test`, `regression test`, `api discovery` |
+
+## Integrations
+
+None required.
+
+## Settings
+
+- **Base URL** -- Base URL of the API to test
+- **Authentication Type** -- `none`, `bearer`, `api_key_header`, `basic`
+- **Auth Token / API Key** -- Credentials for API authentication
+- **Test Mode** -- `functional`, `regression`, `load`, `security`, `comprehensive`
+- **OpenAPI Spec URL** -- URL to OpenAPI/Swagger spec for auto-discovery
+- **Auto Schedule** -- Run tests on a schedule (default: off)
+- **Test Frequency** -- `hourly`, `daily`, `weekly`
+- **Strict Mode** -- Treat any non-2xx response as failure (default: off)
+
+## Usage
+
+```bash
+librefang hand run apitester
+```