Add task metadata to subagent orchestration

Thread phase, lane, ownership, output contract, and spawn reason through
spawn_agent, app-server state, and the TUI background task panel so subagents
stay minimal-profile while each task remains inspectable and manageable.

Keep /agent focused on profile visibility while /subagents preserves thread
switching, and refresh open background task panels when detached terminals
complete.

Tests:
- just fix -p codex-core
- just fix -p codex-app-server-protocol
- just fix -p codex-tui
- cargo test -p codex-app-server-protocol
- cargo test -p codex-core multi_agent_v2
- cargo test -p codex-tui chatwidget::tests::app_server::down
- cargo test -p codex-tui chatwidget::tests::exec_flow::down
- cargo test -p codex-tui slash_agent_opens_profile_manager_instead_of_thread_picker
- cargo test -p codex-tui slash_subagents_keeps_thread_picker_semantics

Author: LEON-gittech