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.

plugins/echo-memory/README.md

@@ -0,0 +1,14 @@
+# echo-memory
+
+Simple demo plugin that echoes back the user message as a "recalled memory". Useful for verifying the plugin system works end-to-end.
+
+## Hooks
+
+| Hook | Script | Description |
+|------|--------|-------------|
+| ingest | `hooks/ingest.py` | Echoes the user message back as a recalled memory fragment |
+| after_turn | `hooks/after_turn.py` | Acknowledges each conversation turn (no-op demo) |
+
+## Usage
+
+Installed automatically when enabled in agent configuration.