Add configurable LLM timeout support

[tennisleng]

Fixes #58

This PR adds configurable timeout support for LLM requests via the STRIX_LLM_TIMEOUT environment variable, while maintaining 180 seconds as the default.

Changes

• Added resolve_timeout() helper function in strix/llm/config.py to read timeout from environment variable
• Updated LLMConfig to accept and store timeout configuration
• Updated LLM._make_request() to use configured timeout instead of hardcoded 180s
• Updated MemoryCompressor to use configured timeout for summarization requests
• Added documentation in README.md and CONTRIBUTING.md

Testing

• All code changes have been verified with linters (no errors)
• Note: The repository currently has no test suite, so pytest fails on coverage requirements