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/test-engineer/README.md

@@ -0,0 +1,23 @@
+# Test Engineer
+
+Quality assurance engineer. Designs test strategies, writes tests, validates correctness.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Module | `builtin:chat` |
+| Model | `default` |
+| Provider | `default` |
+
+System prompt instructs behavior-driven testing philosophy with unit, integration, property, edge case, and regression test design using Arrange-Act-Assert pattern.
+
+## Skills
+
+- `file_read`, `file_write`, `file_list`, `shell_exec`, `memory_store`, `memory_recall`
+
+## Usage
+
+```bash
+librefang agent run test-engineer
+```