fix(pinned): restore latest context usage

[H-TTTTT]

Summary

• restore pinned context usage from the latest non-summary, non-zero assistant measurement by creation time
• prevent stale pre-compaction peaks from being shown after session restore while preserving total session cost
• add regression coverage for out-of-order history, summary messages, and trailing zero-token messages

Testing

• npm run lint
• npm run build
• BOT_LOCALE=en npm test (122 test files, 1089 tests)