feat: Add async and concurrency guidelines to ADK Style Guide

The new guidelines require all I/O operations to be in async functions, prohibit blocking the event loop, and mandate wrapping synchronous I/O calls with asyncio.to_thread.

Co-authored-by: Bo Yang <ybo@google.com>
PiperOrigin-RevId: 930825627

Author: boyangsvl

.agents/skills/adk-style/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: adk-style
-description: ADK development style guide for routine nits — Python idioms, codebase conventions, imports, typing, Pydantic patterns, formatting, logging, and file organization. Use this skill whenever writing code, tests, or reviewing PRs for the ADK project to ensure compliance with styling and coding conventions. Triggers on "code style", "how should I format", "naming convention", "lint", "nit", "imports", "typing", "Pydantic patterns", "testing rules".
+description: ADK development style guide for routine nits — Python idioms, codebase conventions, imports, typing, Pydantic patterns, formatting, logging, async/concurrency, and file organization. Use this skill whenever writing code, tests, or reviewing PRs for the ADK project to ensure compliance with styling and coding conventions. Triggers on "code style", "how should I format", "naming convention", "lint", "nit", "imports", "typing", "Pydantic patterns", "testing rules", "async", "io".
 ---
 
 # ADK Style Guide
@@ -13,6 +13,7 @@ description: ADK development style guide for routine nits — Python idioms, cod
 - [Formatting](references/formatting.md) — indentation, line limits, and running pre-commit hooks.
 - [Documentation](references/documentation.md) — comments and docstrings.
 - [Logging](references/logging.md) — lazy evaluation and log levels.
+- [Async and Concurrency](references/async.md) — async I/O requirements, avoiding blocking the event loop.
 - [File Organization](references/file-organization.md) — file headers and class organization.
 
 ## Testing

.agents/skills/adk-style/references/async.md

@@ -0,0 +1,19 @@
+# Async and Concurrency Style Guide
+
+-   **All I/O operations must be in async functions**: Any operation that
+    performs I/O (network calls, file system access, database queries, etc.)
+    must be defined in an `async def` function.
+-   **Do not block the event loop**: Avoid calling blocking synchronous
+    functions directly from async code.
+-   **Wrap synchronous I/O**: If you must use a synchronous library for I/O
+    (e.g., standard `open()`, `pathlib` file operations, or synchronous
+    clients), wrap the blocking call in `asyncio.to_thread` to run it in a
+    separate thread and prevent blocking the main event loop.
+
+Example:
+
+```python
+async def save_data(path: Path, data: bytes) -> None:
+  # Wrap blocking file write in asyncio.to_thread
+  await asyncio.to_thread(path.write_bytes, data)
+```