Generic Refactoring Rules for Sourcegraph Cody

# Generic Refactoring Rules for Sourcegraph Cody

## Safe refactors
- Preserve behavior first; prove with tests.
- Refactor in small commits with clear intent.
- Keep public interfaces stable unless explicitly versioning changes.

## Code health
- Remove duplication by extracting domain-level abstractions.
- Prefer explicit composition over inheritance chains.
- Replace primitive obsession with value objects/types where useful.

## Reviewability
- Add migration notes when refactor touches data flow or side effects.
- Include before/after perf notes if touching critical paths.