#8244: feat(hooks): add session:before_compact and session:after_compact internal hook events

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: ```typescript 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_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> 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. <h3>Confidence Score: 4/5</h3> - 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 <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->