#12046: fix(compaction): add fallback for undefined ctx.model (#12016)

## Problem Compaction safeguard fails when `ctx.model` is undefined, causing exit code 1 and falling back to generic summary instead of generating AI-powered summary. ## Solution - Changed `const model` to `let model` to allow reassignment - Added fallback logic to retrieve model from `getCompactionSafeguardRuntime()` - Added comprehensive diagnostic logging for debugging ## Benefits - Prevents compaction failures when model context is missing - Provides detailed error logs for troubleshooting - Maintains backward compatibility with existing code Fixes #12016 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates the compaction safeguard extension to be more resilient when `ctx.model` is missing. It introduces a fallback to read the model from the compaction safeguard runtime (via `getCompactionSafeguardRuntime(ctx.sessionManager)`), and adds additional warning/error logging to aid diagnosis. This keeps the existing behavior of returning a generic fallback summary when no model/API key is available, but avoids failing compaction purely due to an undefined model in the compaction context. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge, with one logging/data-exposure concern to address. - The functional change is small and contained (model fallback), and preserves the existing fallback-summary behavior when no model/API key is available. The main remaining risk is the new diagnostic logging potentially emitting sensitive internal context details into logs when `ctx.model` is absent. - src/agents/pi-extensions/compaction-safeguard.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 -->