Summary

Closes #65220.

This teaches context engines to opt into background turn maintenance and moves turn-triggered maintenance off the foreground session lane when turnMaintenanceMode === "background". For lossless-claw, that means proactive post-turn maintenance can be queued as a hidden task instead of blocking the next user turn.

Why

• Today the host awaits both afterTurn() and maintain(... reason: "turn") inline on the session path.
• Even if a plugin removes heavy foreground work, the host still controls whether turn maintenance blocks the lane.
• The result is visible assistant output followed by dead time before the next command can start.
• Subagent completion announce traffic can time out behind a stalled main lane.

What Changed

• Added turnMaintenanceMode?: "foreground" | "background" to context-engine info.
• Added allowDeferredCompactionExecution?: boolean to runtime context.
• When a context engine opts into background mode, runContextEngineMaintenance(... reason: "turn") now:
  • queues one hidden maintenance task per session key
  • coalesces repeated requests
  • waits for the session lane to go idle before executing
  • runs maintenance with allowDeferredCompactionExecution: true
  • uses task delivery/status infrastructure instead of a visible subagent or announce flow
• Long-running or failed deferred maintenance can surface subtle task updates to the session.
• Fast deferred maintenance stays silent.

Guarantees

• Proactive turn maintenance no longer blocks the immediate next foreground turn for engines that opt in.
• Overflow/timeout/manual compaction remains synchronous in the plugin/runtime paths that require correctness.
• Hidden maintenance is not a child agent run and does not create synthetic subagent sessions.

Validation

• pnpm test -- --run src/agents/pi-embedded-runner/context-engine-maintenance.test.ts src/infra/backoff.test.ts passed
• NODE_OPTIONS=--max-old-space-size=8192 pnpm exec tsc -p tsconfig.json --noEmit still reports preexisting unrelated AbortSignal type errors in unchanged extensions/telegram/src/bot.ts

Companion Plugin Work

• Companion lossless-claw issue: Proactive threshold compaction can stall foreground turns and hide maintenance state Martian-Engineering/lossless-claw#407
• Companion lossless-claw PR: Defer proactive compaction debt and surface LCM maintenance state Martian-Engineering/lossless-claw#408