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/social-media/README.md

@@ -0,0 +1,23 @@
+# Social Media
+
+Social media content creation, scheduling, and engagement strategy agent.
+
+## Configuration
+
+| Field | Value |
+|-------|-------|
+| Module | `builtin:chat` |
+| Model | `default` |
+| Provider | `default` |
+
+System prompt instructs platform-optimized content creation, content calendar planning, engagement strategy, analytics interpretation, and brand voice consistency.
+
+## Skills
+
+- `file_read`, `file_write`, `file_list`, `memory_store`, `memory_recall`, `web_fetch`
+
+## Usage
+
+```bash
+librefang agent run social-media
+```