feat: batch background notifications

[tt-a1i]

Summary

• batch /cron jobs due in one polling tick into one structured follow-up and one triggered model turn
• coalesce idle background-terminal settlements in one fixed 200 ms window while preserving the existing quiet busy-backlog path
• retain exactly-once consumption, atomic cron retry, immediate bg_watch, shutdown cleanup, and truthful multi-terminal rendering

Why

Near-simultaneous build, test, lint, and package completions could wake the model once per process while idle. Multiple cron jobs due in one tick likewise queued separate triggered turns. One bounded batch gives the model a complete decision set with less context fragmentation and fewer model turns.

Safety boundaries

• the background window is fixed, not sliding, so delivery cannot be postponed indefinitely
• bg_start reserves against running, pending, and concurrent starts so the retractable exactly-once backlog stays bounded without evicting identities
• a busy race retains results for agent_settled and delivers them as nextTurn without forcing another response
• timer cancellation uses an opaque token, making already-queued stale callbacks harmless
• bg_status / bg_kill consumption and the id-keyed delivery map remain authoritative
• bg_watch uses its existing independent immediate message path
• cron delivery is atomic: if sendMessage throws, no due job advances
• no scheduler persistence, model scheduling tool, child capability, or new parent wake mechanism

Test plan

• TDD regressions for cron same-tick batching and atomic retry
• TDD regressions for fixed idle window, busy race, stale timer/shutdown cleanup, and multi-result rendering
• focused batching tests: 28/28
• background-terminal and cron suites: 85/85
• full Node suite: 605/605
• Vitest suite: 29/29
• Biome format and lint
• TypeScript / Effect typecheck (existing unrelated Effect warnings only)
• git diff --check
• npm pack/install and packed Pi extension load: 121 files; tests excluded

Closes #8