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/hooks/README.md

@@ -0,0 +1,15 @@
+# echo-memory hooks
+
+Python hook scripts for the echo-memory demo plugin. Each script reads a JSON request from stdin and writes a JSON response to stdout.
+
+## Scripts
+
+| Script | Hook | Description |
+|--------|------|-------------|
+| `ingest.py` | ingest | Receives `{"message": "..."}`, returns it wrapped as a memory fragment |
+| `after_turn.py` | after_turn | Receives the conversation messages array, responds with `{"type": "ok"}` |
+
+## Protocol
+
+- **Input**: JSON object on stdin (fields vary by hook type)
+- **Output**: JSON object on stdout (`ingest_result` with memories, or `ok`)