Add LangGraph adapter, tests, and bump to 0.5.0

[nikhilvdev]

Introduce logquill.adapters.langgraph: LangGraphAdapter extends LangChainAdapter and GraphCallbackHandler to map LangGraph checkpoint events (on_interrupt → observation 'graph_interrupted', on_resume → action 'graph_resumed') into LogQuill records. Adds actionable ImportError when langgraph isn't installed. Add tests for the adapter, update README and CHANGELOG with LangGraph docs, add optional 'langgraph' extra in pyproject.toml, and bump package version to 0.5.0 (pyproject + init).

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope