feat(agentlayer): add resilient context compaction

[K-Mistele]

Summary

• add explicit and automatic context compaction for root and child agents, including /compact, proactive thresholds, and one-shot overflow recovery
• retain a safe recent message tail, support Pi-style split-turn summaries, persist checkpoint metadata, and reset stale filesystem bookkeeping
• expose typed compaction events, atomic state updates, configurable initial/incremental prompts, bounded summary output, and exact aggregate usage accounting

Verification

• bun check (1,269 passed, 6 skipped, 0 failed)
• focused compaction, state, child isolation, streaming event, and filesystem hook tests

Follow-up

• publish an AgentLayer release from this PR for Synclayer Phase 5 integration

[github-actions]

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