feat: add Pi-native context compaction

[sambitcreate]

Summary

• adopt Pi 0.80.10 native semantic compaction for primary chats and child agents
• persist private append-only Pi JSONL journals and reconstruct context from native checkpoints plus retained tails
• compact proactively near the model limit and compact/retry exactly once after context overflow
• keep the existing deterministic context transform as the emergency payload-safety layer
• expose bounded compaction activity, disclose local compaction history during onboarding, and delete journals with chats

Pi parity

The implementation delegates cut points, structured summary prompting, repeated-summary updates, split-turn handling, tool-pair safety, token estimates, and retained-tail reconstruction to the pinned Pi core package. Both Agent paths install Pi's harness message converter so native compactionSummary checkpoints reach subsequent provider requests.

Verification

• npm test — 1,185 JavaScript tests, 33 native worktree-remover tests, and 41 Rust broker tests passed
• npm run type-check
• npm run lint
• npm run build
• focused compaction, child-agent continuation, timeline, onboarding, and message-rehydration suites
• git diff --check

Base

This is intentionally stacked on features-jul30; that branch contains the current subagent runtime that receives the matching child-agent compaction path.