Protect concurrent session cookie saves

[ryanfowler]

Summary

• Add a per-session advisory lock around session file saves
• Reload the latest session JSON before writing and merge local cookie creates, updates, and deletions
• Cover the interleaved-save case with regression tests so distinct cookies survive concurrent --session runs

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