Kill Legacy

Single goal: eliminate legacy and fallback code so the codebase has one current path, not old + new forever.

Do not broaden into general smell cleanup, renaming, or unrelated refactors. If code is not legacy/fallback (or required to finish removing it), leave it alone.

Core Rules

• Prefer deleting the old path over wrapping, flagging, or “cleaning up later.”
• The current implementation wins. Do not preserve dead dual paths “just in case” without evidence they are still required.
• Preserve public behavior and external contracts unless the user authorizes a breaking change. When a legacy path is the public contract, migrate callers or get explicit approval before removal.
• Prove a path is unused or replaceable before deleting it (references, feature flags, env defaults, runtime config, docs, tests).
• Remove the scaffolding that kept legacy alive: flags, env vars, adapters, dual-read/dual-write, temporary branches, and tests that only exist for the old path.
• Do not replace a removed fallback with a new silent fallback. Fail loudly at the boundary if input is invalid for the current model.
• Preserve unrelated user changes. Keep passes reviewable.

Establish Scope