Pass user query context through proxy pipeline (9 call sites)

Fix test failures: update hash test for MD5, handle unicode surrogates

Reduce proxy latency, enrich telemetry, protect Bash output, bump to 0.5.6

Fix CI: guard starlette imports, asyncio.run(), deprecate datetime.utcnow()

Fix all mypy type errors and flaky embedding test

Initial commit: Headroom SDK - LLM context optimization toolkit