Closes #3439

Summary

Adds a team-level live-session view to /context and explicit compaction targeting for individual sub-agent sessions.

The runtime tracks active RunStream sessions, including foreground delegation and run_background_agent work. Targeted requests are queued onto the selected session's own run loop and execute between model turns, preventing compaction from racing an in-flight turn.

Issue expectations

Runtime flow

/context
  -> LiveSessions(current root)
  -> select live row
  -> CompactLiveSession(session ID)
  -> bounded request queue on target live-session entry
  -> target RunStream reaches a safe iteration boundary
  -> manual compaction events return through the App event stream

Accepted requests are drained before stream teardown. Requests on already-cancelled streams complete as skipped, avoiding a misleading started/failed sequence. Cached root-tree identity keeps long-running nested background sessions visible after an intermediate foreground parent finishes, while excluding stale sessions from a replaced root.

TUI behavior

• Up/Down moves across live-session and attached-file rows.
• Enter compacts a selected live session.
• d, x, or Delete still drops a selected attachment.
• The current/root row preserves the existing /compact path.
• Targeted child compaction does not cancel the root stream or alter its working state.
• The sidebar compaction indicator only follows the session currently displayed.

Design decisions and residual scope

• Live-session listing and targeting are optional local-runtime capabilities, matching the existing optional context-breakdown pattern. Remote runtimes omit the live-session section and report targeted compaction as unsupported.
• Harness-backed sessions have no regular model-loop iteration boundaries and no known context limit. An accepted request is therefore handled during teardown rather than interrupting harness execution.
• Existing proactive threshold and overflow-recovery compaction remain active for sub-agent sessions. This change only adds explicit cross-agent orchestration.

Validation