feat(runtime): cross-agent compaction orchestration (explicit, user-triggered)

[aheritier]

Summary

Provide a team-level view of every live session's context budget and let the user trigger compaction on a specific sub-agent session (especially long-running background agents). A multi-agent differentiator no competitor has.

Proposed behavior

• Team-level view (extend /context) listing every live session's budget.
• Ability to trigger compaction on a specific named sub-agent session.
• Decision: explicit user action only for now. Idle auto-compaction of background sessions is explicitly deferred (highest blast radius).

Implementation notes (starting points)

• pkg/runtime — support compaction targeting a named session
• pkg/runtime/agent_delegation.go, pkg/tools/builtin/agent/agent.go
• TUI dialogs
• ⚠️ Touches the delegation event-forwarding paths — highest blast radius; keep behind explicit user action.

Complexity: L.

Background: shared plan context-management-gap-analysis (Phase 3, item 8).

Dependencies / Relationships

• Blocked by: feat(tui): add /context command with a categorized context-window breakdown #3432 (extends /context into a team-level view), feat(tui): per-sub-agent context accounting in the TUI #3438 (builds on per-sub-agent accounting)
• Related: feat(config): make the auto-compaction threshold configurable (compaction_threshold) #3433 (configurable compaction)