Adds internal hook events that fire before and after session compaction, allowing hooks like memory-extract to capture session content before it's summarized.

Events

• session:before_compact: Fires before session.compact() with context: sessionFile, sessionId, cfg
• session:after_compact: Fires after compaction with context: sessionFile, sessionId, tokensBefore, tokensAfter, cfg

Behavior

• before_compact is awaited to ensure hooks can capture session content synchronously before compaction modifies/summarizes it
• after_compact is fire-and-forget to not block the compaction return

Use Case

Memory extraction hooks can now listen for session:before_compact to capture conversation facts before context is compacted, preventing information loss during auto-compaction.

Example hook handler:

if (
  (event.type === 'command' && event.action === 'new') ||
  (event.type === 'session' && event.action === 'before_compact')
) {
  const content = fs.readFileSync(event.context.sessionFile, 'utf-8');
  // Extract facts from content before compaction...
}

Greptile Overview

Greptile Summary

This PR adds two new internal hook events around session compaction in the embedded runner (session:before_compact awaited; session:after_compact fire-and-forget) and documents them in the bundled hooks README. The events include session identifiers and compaction token stats so bundled/custom hooks (e.g., memory extraction) can capture session state before summarization alters it.

Confidence Score: 4/5

• This PR is generally safe to merge; it’s a small, localized behavior change adding hook emissions around compaction.
• Changes are confined to emitting internal hook events and updating documentation. Main risk is semantic/contract mismatch for hook consumers (e.g., after_compact not firing on failures), but there are no obvious runtime/type issues introduced.
• src/agents/pi-embedded-runner/compact.ts

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