fix(pool): hung eviction drops the session's [[ws:]] workspace #1556

Description

When a session is force-evicted as hung, purge_session_entries() also removes its session_workdirs entry (crates/openab-core/src/acp/pool.rs, added in #1300). The next session for that thread starts in the default working_dir, not the [[ws:...]] workspace.

Steps to Reproduce

1. Start a session with [[ws:@alias]] ... in a thread.
2. Let one turn run past prompt_hard_timeout_secs + hung_grace_secs (log: force-evicting hung session … threshold_secs=1920).
3. Send another message to the thread without a directive (for us: a usercron job with thread_id).
4. The new agent is spawned with cwd = default working_dir.

Seen twice on 2026-10-01 on 0.10.0-beta.4 with claude-agent-acp.

Expected Behavior

The workspace survives eviction, as documented in docs/workspaces.md ("persists across session suspend/resume and eviction rebuilds") and docs/adr/control-directives.md §3.1. Only an explicit reset should clear it.

I will send a PR.