cleanup-legacy

Find code marked deprecated/legacy/old/v1 and verify it's truly unused before deletion. Also find unreachable fallback branches (e.g., feature flag defaults that have flipped, version checks for unsupported runtimes).

Preflight

1. Language detect — applies to all.
2. Git state: refuse on dirty tree.
3. Report dir: ensure exists.
4. Read deprecation markers the project uses. Common ones:
   • @deprecated JSDoc/TSDoc
   • # deprecated Python comments, warnings.warn(DeprecationWarning)
   • // Deprecated: Go convention
   • #[deprecated] Rust attribute
   • File/dir naming: legacy/, old/, v1/, _old.ts
5. Read feature flag config if present — flags that are 100% on with no opposite tests can have their else branches deleted.

Detect