Refactor shared file lock handling

[ryanfowler]

Summary

• Centralize cross-platform advisory lock open/retry/unlock logic in src/fileutil.rs with a reusable FileLock helper
• Switch session and update lock paths to use the shared helper while preserving their existing timeout and warning behavior
• Add regression tests for nonblocking acquisition, timeout mapping, contention hooks, and successful blocking acquisition after release
• Update AGENTS.md to document the shared lock owner

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1