Pi compaction events now carry reason and willRetry so a harness can tell the trigger apart

What this changes for operators

• v0.79.10 (PR #5962) adds reason and willRetry to the extension session_before_compact and session_compact events, so an extension can distinguish a manual /compact from a threshold auto-compaction from an overflow-retry flow. The runtime's context management becomes legible from the outside.
• Harness authors building on Pi extensions can now branch behavior on why compaction fired and whether it will retry, rather than inferring it.