#9425: refactor(agents): replace console.warn with SubsystemLogger in compaction.ts

## Description Refactors `src/agents/compaction.ts` to use the standardized `SubsystemLogger` instead of direct `console.warn` calls. ## Motivation This improves logging consistency across the codebase. ## Changes - Replaced `console.warn` with `log.warn`. - - Initialized `createSubsystemLogger("agents/compaction")`. ## Tests - Verified with existing tests: `npx vitest run src/agents/compaction.test.ts` (Passed). <!-- greptile_comment --> <h3>Greptile Summary</h3> Replaces two `console.warn` calls with `log.warn` using the standardized `SubsystemLogger` in `src/agents/compaction.ts`. - Adds import for `createSubsystemLogger` from `../logging/subsystem.js` - Initializes logger with `createSubsystemLogger("agents/compaction")` - Updates two error fallback paths in `summarizeWithFallback` function to use `log.warn` instead of `console.warn` This change aligns with the codebase's logging conventions - multiple other files in `src/agents/` already follow this pattern (e.g., `tool-loop-detection.ts`, `tool-images.ts`, `cli-credentials.ts`). <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - The change is a straightforward refactor that replaces direct `console.warn` calls with the standardized `SubsystemLogger.warn` method. The implementation follows established patterns used throughout the `src/agents/` directory, maintains identical functionality, and only affects logging output formatting. No logic changes or behavioral modifications are introduced. - No files require special attention _{Last reviewed commit: 38710fb} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->