poll_sessions uses blocking std::fs I/O inside tokio::spawn — holds the worker thread during session file scans

[ooloth]

Current state

poll_sessions is called from inside tokio::spawn in the TUI's stream_interval tick, but it performs blocking filesystem I/O (std::fs::read_dir and std::fs::read_to_string) synchronously on the tokio worker thread. Hub's Rust conventions require using tokio::fs rather than std::fs equivalents inside async contexts. The scan visits every file in the sessions directory for every in-progress task on every tick, holding the worker thread for the full duration of the scan.

Ideal state

• poll_sessions uses tokio::fs::read_dir and tokio::fs::read_to_string (making read_session_snapshot async), or the call site in main.rs wraps poll_sessions in tokio::task::spawn_blocking
• The tokio worker thread is never held while reading session files from disk

Starting points

• workflows/src/agent_session.rs lines 174–199 — read_session_snapshot contains the blocking std::fs::read_dir (line 178) and std::fs::read_to_string (line 195)
• ui/tui/src/main.rs line 295 — the tokio::spawn call site that invokes poll_sessions

QA plan

1. Open workflows/src/agent_session.rs — confirm read_session_snapshot is not async fn and uses std::fs::read_dir at line 178 and std::fs::read_to_string at line 195
2. Open ui/tui/src/main.rs line 295 — confirm poll_sessions is called inside tokio::spawn(async move { ... }) without spawn_blocking
3. After the fix: confirm either read_session_snapshot uses tokio::fs equivalents and poll_sessions is async fn, or the call site in main.rs wraps the call in tokio::task::spawn_blocking
4. Run just check — expect no warnings or errors
5. Run just test — expect all tests to pass

Done when

poll_sessions does not hold the tokio worker thread while reading session files from disk.