fix: correctly bind async local plugins as coroutines

[akshatmalik-bruh]

Description

Problem

Currently, a local plugin implemented as async def is accepted during the engine build but never actually executes. The engine records the tool call as succeeded and hands the LLM a stringified coroutine object instead of the actual resolved value. This causes a P2 silent no-op.
fixes #141

The root cause was that GraphBuilder._build_agent_tools was passing all functions through StructuredTool.from_function(function). This registers async functions in the synchronous func slot, which LangChain's synchronous execution path returns instead of awaiting.

Solution

Updated GraphBuilder._build_agent_tools to properly detect async functions using inspect.iscoroutinefunction(function).

• If it is a coroutine, it registers it using coroutine=function.
• If it is synchronous, it continues to use func=function.

Testing and Verification

• Added an executable regression test (tests/test_review_regression.py) that mocks a model and verifies that the async def body physically executes and writes to disk.
• Existing synchronous local tools continue to work perfectly.
• Ran make check and verified that formatting, linting, type-checking, and all tests pass locally.