chore: Remove duplicate task_inheritance.rs from turborepo-lib (#11347)
## Summary
- Delete orphaned `crates/turborepo-lib/src/engine/task_inheritance.rs`
(574 lines)
## Details
After the engine extraction in #11346, `task_inheritance.rs` was left
behind as a duplicate. The canonical `TaskInheritanceResolver`
implementation now lives in `turborepo-engine/src/builder.rs` and is
exported from the crate.
The file was never referenced (no `mod task_inheritance;` declaration
existed in `mod.rs`) and was completely orphaned.
**What remains in `turborepo-lib/src/engine/`:**
- `mod.rs` (647 lines): `EngineExt` trait, `ValidateError`, re-exports -
lib-specific validation logic
- `builder.rs` (3,197 lines): Integration tests that require
lib-specific types (`TurboJsonLoader`, `TurboJson`)
## Testing
- `cargo check -p turborepo-lib` ✅
- `cargo test -p turborepo-lib --lib` ✅ (313 tests)
- `cargo test -p turborepo-engine` ✅ (4 tests)