Update engineer agent instructions to enforce autonomous review-fix loop

- Mandate immediate resolution of all reviewer findings (BLOCKERs, WARNINGs, and NITs) without user intervention.
- Clarify that the 3-fix escalation rule applies to implementation, while the review cycle has a separate 3-iteration cap.
- Revise tech debt guidelines to prioritize performing the "right fix" autonomously rather than asking for permission.
- Add stall prevention rules to ensure the agent continues working until a clean review is achieved or the limit is reached.

Author: niemyjski