fix(tui): keep Desktop sessions open across gateway reload

[HexLab98]

Summary

• Desktop backend reloads (profile switch, WS 1012 service restart) were tearing down live chats as if the user had closed them: process-exit finalize stamped tui_shutdown, and the disconnect path still armed the WS-orphan reap against detached sessions (reaped_sessions=0, detached_sessions=N).
• Populated Desktop/TUI rows now stay open through that bounce so the next process can list and resume them. tui_shutdown is treated as a recoverable accidental end for rows already stamped by older builds.
• delete_session / delete_session_if_empty log id, reason, and whether the row had messages, so a silent hard-delete is greppable.

Fixes #95868

Test plan

• scripts/run_tests.sh tests/tui_gateway/test_finalize_session_persist.py tests/test_empty_session_hygiene.py tests/tui_gateway/test_startup_orphan_sweep.py tests/test_tui_gateway_ws.py
• scripts/run_tests.sh tests/test_tui_gateway_server.py tests/tui_gateway/test_session_reclaim_notify.py -k "close_sessions_for_transport or test_session_delete or reclaim"
• Desktop: open several multi-turn chats, switch profiles (watch for ws closed ... code=1012 ... detached_sessions>0), switch back, confirm the original chats still appear and resume with history intact
• sqlite3 state.db "SELECT id, ended_at, end_reason FROM sessions;" after the reload — pre-reload ids still present, ended_at null for the bounced chats
• Explicit sidebar delete still removes the row and logs delete_session id=... reason=session.delete (or api.sessions.delete)

[teknium1]

Closing as superseded on main. The delete vector this PR targeted (non-empty Desktop session rows being hard-deleted during a gateway reload, #95868) was fixed on main through a different design:

• 0bfe715e4d (PR fix(sessions): don't let the empty-session sweep delete an archived transcript #96092) hardened the empty-session sweep — _EMPTY_SESSION_WHERE now requires ended_at, archived = 0, AND a NOT EXISTS messages probe (citing Desktop: active non-empty sessions silently hard-deleted from sessions table during gateway reload (detached_sessions>0, no delete_session call logged) #95868), so a non-empty row can no longer be swept.
• 39f1e1881a added _AUTOMATIC_SESSION_END_REASONS with lease-guarded row preservation, so automatic Desktop ends (tui_shutdown, ws_orphan_reap) keep state.db rows when another backend holds the session.

Your alternative (never stamping tui_shutdown, treating it as recoverable) now conflicts semantically with main's lease-guard architecture, and the branch is ~880 commits behind with conflicts in every core file. Your diagnosis of the reload delete vector was correct and early — thank you. One still-useful piece of this PR is audit logging on delete_session/delete_session_if_empty (id/reason/had-messages); if you'd like to resubmit that as a small fresh PR against current main, it would be welcome.