Prevent unrelated edits outside the user's prompt scope

[linear-code]

Problem

The Evolve agent must not make tangential or “helpful” changes that the user did not ask for. This is broader than preserving Nix formatting: if the user asks for one change, the resulting diff should stay scoped to that prompt.

This preserves the distinct acceptance criterion from ENG-210 while moving it under the broader Evolve quality eval suite.

Acceptance Criteria

• Representative eval cases include prompts where the correct behavior is a tightly scoped diff.
• The eval output classifies unrelated edits / prompt-scope violations separately from formatting, protected-file, and normal correctness failures.
• Launch-blocking threshold: zero severe cases where a simple user prompt produces unrelated package, shell, config, or structural changes.
• A post-evolution validation approach is specified: either prompt-level constraint, deterministic diff-vs-prompt checks, reviewer rubric, or a combination.
• Any implementation path keeps the distinction clear between:
  • unrelated edits outside the prompt scope
  • formatting/comment preservation regressions covered by [Launch blocker] Preserve existing Nix code formatting & structure during package installs #137
  • protected-file enforcement covered by ENG-236

Source

Supersedes ENG-210's unique diff-scope requirement without keeping the old “Trust & Safety” framing.