Background brief synthesis avoids MCP timeouts by running as a fire-and-forget
task launched with `asyncio.create_task`, meaning `_background_brief_synthesis()`
is called without awaiting or returning its result. This decouples the slow
LLM-based brief generation from the MCP request lifecycle, so the IDE agent
receives its response immediately rather than waiting for synthesis to complete.

Sources:
* main.py:784 - 1.96 (rerank)
* main.py:1827 - 0.99 (rerank)
* main.py:393 - 0.98 (rerank)
* main.py:2094 - 0.94 (rerank)