#17639: fix: Memory indexer skips session files

## Summary When the MemoryManager is constructed, the `dirty` flag is set to true for memory sources to ensure files get indexed on first sync. However, the `sessionsDirty` flag was never initialized, causing session files to never be indexed unless `--force` was used or session files changed externally. This fix adds the same initialization pattern for `sessionsDirty`: it is set to true when sessions is in sources (for non-status operations). This ensures session files are indexed on first sync, matching the behavior of memory files. ## Changes - Added initialization of `sessionsDirty` flag in `MemoryManager` constructor: `this.sessionsDirty = this.sources.has("sessions") && (statusOnly ? !meta : true);` ## Testing - All existing memory tests pass: index.test.ts, manager.async-search.test.ts, session-files.test.ts, memory-cli.test.ts - The fix follows the same pattern as the existing `dirty` flag initialization for memory files Fixes openclaw/openclaw#17576 <!-- greptile_comment --> <h3>Greptile Summary</h3> Initialized `sessionsDirty` flag in `MemoryManager` constructor to match the existing `dirty` flag pattern, ensuring session files are indexed on first sync when sessions source is configured. - **What Changed**: Added initialization of `sessionsDirty` flag following the same logic as `dirty` flag initialization - **Why This Matters**: Without this initialization, session files were never indexed on first sync unless `--force` was used or files changed externally - **Pattern Match**: The fix correctly mirrors the existing `dirty` flag pattern: `this.sources.has("sessions") && (statusOnly ? !meta : true)` - **Integration**: The `sessionsDirty` flag is properly used throughout the sync logic in `manager-sync-ops.ts` via `shouldSyncSessions()` method <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The fix is a single-line addition that follows the exact same pattern as the existing `dirty` flag initialization. The logic is correct, consistent with the existing codebase, and addresses a clear bug where session files were skipped on first sync. The change integrates seamlessly with existing sync logic that already checks `sessionsDirty` throughout the codebase. - No files require special attention. _{Last reviewed commit: be1605f} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->