Add close() to GraphitiClient and make HTTP timeout configurable

[hocinehacherouf]

Problem

GraphitiClient has two issues:

1. Connection leak — The httpx.AsyncClient is created lazily but never closed. Hooks (session_start, session_end, _ingest_worker) create a new client per invocation and never release connections.

2. Hardcoded timeout — The HTTP timeout is hardcoded to 60s with no way to override it. Users behind slow networks or corporate proxies may need to tune this.

Proposed changes

• Add timeout parameter to GraphitiClient.__init__
• Add close() method and async context manager (async with) support
• Add GRAPHITI_TIMEOUT env var to Settings
• Update all hooks to use async with GraphitiClient(...) as client:
• Add unit tests for timeout, close, and context manager