fix: reap dashboard and TUI child processes

[solovision24]

Summary

• add dashboard action waiter threads to reap completed subprocesses and close inherited log handles
• preserve latest dashboard action status after removing exited Popen handles
• add TUI slash-worker waiter thread so exited slash workers do not remain defunct

Test Plan

• python -m pytest tests/hermes_state/test_repair_stale_open_sessions.py tests/hermes_cli/test_web_server.py::test_dashboard_action_waiter_reaps_completed_child tests/test_tui_gateway_server.py::test_slash_worker_waiter_reaps_exited_process tests/cli/test_single_query_closeout.py -q -o 'addopts='\n- python -m pytest tests/hermes_cli/test_web_server.py tests/test_tui_gateway_server.py tests/hermes_state/test_repair_stale_open_sessions.py tests/cli/test_single_query_closeout.py -q -o 'addopts='\n\n## Notes\n- Existing zombies under already-running dashboard/TUI processes require a service restart to disappear; this patch prevents new leaks once deployed.

[alt-glitch]

Competes with #22863 (robust slash_worker lifecycle management). Both address the slash_worker zombie leak (#21370). This PR also adds dashboard action waiter threads (broader scope).

[teknium1]

Superseded by #42132 (merged), which closes the slash_worker subprocess leak via two guards: process-group kill on PTY teardown + a cross-platform parent-death watchdog in the worker. Closing as resolved — thanks for tackling this; the merged fix salvaged the process-group-kill and watchdog approaches with contributor authorship preserved.