Stop later Online Context Compact boundaries from inheriting a stale request horizon

[YPQuinn]

Summary

• Fixes later-session Online Context Compact over-triggering (Online Context Compact over-triggers later boundary compactions from a stale request horizon #70): compaction epochs now drop completedBoundaryRequestCounts, persist the last real summary size, and defer near-floor, cooldown, and unrepaid cache-debt cases.
• A completed plan plus a new user turn starts a fresh request horizon so a short follow-up task does not inherit the research-phase cadence.
• Window protection is unchanged. Regression fixtures keep the first long-session compact and an epoch-local mid-session compact economic, and require the 40K wrap-up / short follow-up cases to defer.

Test plan

• npx vitest run tests/online-context-compact-economics.test.ts tests/online-context-compact-state.test.ts tests/online-context-compact.test.ts tests/online-context-compact-agent-session.test.ts
• npm run typecheck
• npx vitest run (151 tests)
• Confirm a long research-then-implement session no longer chains a compact 4 minutes after the previous one at ~40K context
• Confirm a new user task after the previous plan is fully completed does not inherit the old boundary-request mean

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