fix: clean up tmux sessions

[jschwxrz]

issue

• task/project deletion could leave tmux sessions behind
• cleanup relied too much on in-memory PTY state, which can drift

fix

• run task teardown before DB deletion in deleteTask
• track knownSessionIds in conversation/terminal providers so tmux cleanup does not depend only on live PTY map entries
• add DB-backed fallback cleanup in local/ssh project providers for detached/non-provisioned tasks
• make teardown scripts bounded and non-blocking (10s timeout, log and continue), so resource cleanup still completes