Add a touched-code cleanup guardrail

[ssheld]

Summary

• add an explicit scope guardrail so simplification, cleanup, and refactoring stay focused on code already touched by the current task
• state clearly that unrelated cleanup should not be pulled into a change unless it was explicitly requested or is necessary for correctness/maintainability of the current change
• keep this as the first small implementation slice under Proposal: absorb useful code-simplifier patterns into agent-vault without importing Claude-specific style rules #56, without taking on the readability-defaults follow-up yet

Files Changed

• scaffold/agent-vault/shared-rules.md: add the new Core Principles guardrail
• scaffold/agent-vault/AGENTS.md: mirror the shared-rules change

Validation

• bash scripts/check-policy-mirrors.sh
• git diff --check

Risks

• Low risk; this is workflow-policy only and affects future agent behavior, not scripts or generated file structure.

Docs Consistency

• No README.md or docs/design.md changes needed.

Notes

• This is PR 1 of the agreed split on Proposal: absorb useful code-simplifier patterns into agent-vault without importing Claude-specific style rules #56.
• The readability-defaults follow-up remains to be implemented separately.

Refs #56

[ssheld]

🤖 Review by Claude Opus 4.6 · via Claude Code
Formal review state unavailable: cannot approve own pull request

No findings above the confidence threshold.

One line, exactly where we agreed it should go (Core Principles, after the planning guardrail), identically mirrored. This matches the #56 consensus plan — PR 1 of 2.

The wording is well-calibrated. "Do not widen scope into unrelated cleanup unless it was explicitly requested or is necessary for correctness or maintainability of the current change" gives two clear exception paths (user asked for it, or it's needed for the change to be correct/maintainable) without leaving room for "this nearby code could use some cleanup" drift.

Placement as the last bullet in Core Principles is correct — it's a scope discipline rule that applies across all work, not specific to any one workflow phase.

Strengths

• Minimal, focused change that delivers the highest-value import from #56.
• The two exception paths ("explicitly requested" and "necessary for correctness or maintainability") prevent the guardrail from being too rigid while still closing the opportunistic-cleanup loophole.

Merge Recommendation

Approve

Top Risks

• None.

Suggested Additional Tests

• None beyond the mirror check already run.