
# Implementation Rules

## Change policy
- Prefer edits over rewrites.
- Keep changes localized.
- One change = one purpose.
- Avoid unnecessary abstraction.

## Dependency policy
- Default: do not add dependencies.
- If adding: explain why, alternatives considered, and impact.

## Error handling
- Validate inputs at boundaries.
- Error messages must be actionable: what failed + what to do next.
