Partial edits lack follow-up signals

### Description

Direct customer feedback:

> During upgrades, the agent sometimes leaves partial edits (e.g., commenting out a few lines, rewriting part of a method) without any follow-up signal — no TODO, no warning comment, no build-time diagnostic. This makes the upgrade appear complete, allowing subtle runtime issues to slip through review.

Expected Behavior

The agent should leave a clear marker (e.g., // TODO: [Modernize] Verify this migration — original code was commented out) whenever it cannot fully resolve a pattern, so developers know which areas need manual verification.