refactor: Port Cargo task and contract knowledge (#13581)
## Intent
Move Cargo task commands, entrypoint selection, defaults, derived
inputs, environment, exact outputs, and cache safety into immutable
repository knowledge.
**Base:** `main`.
## Changes
- Retain Cargo override framing and serial grouping in native-task scope
knowledge.
- Remove dead command-era package-manager and graph resolution helpers.
- Produce immutable Cargo task contracts during discovery.
- Compose cache defaults, dependency-source inputs, hashed environment,
output layout, and safety states from `PackageTaskContext`.
- Move task entrypoint selection from mutable `CargoToolchain` state to
immutable contracts.
- Preserve query task enumeration and exclude-only Cargo entrypoint
selection.
- Project task-I/O environment patterns from contract knowledge.
- Delete task defaults, task-I/O, and entrypoint-selection callbacks
from `Toolchain`.
- Preserve fail-closed output and untracked-input behavior.
## Testing
- `cargo test -p turborepo-repository cargo::test` (41 passed)
- `cargo test -p turborepo-engine` (126 passed)
- `cargo test -p turbo --test cargo_workspace_test` (57 passed)
- `cargo clippy -p turborepo-repository -p turborepo-engine -p
turborepo-lib -p turborepo-task-executor -p turborepo-run-summary
--all-targets -- -D warnings`
- `cargo fmt --check`
Closes TURBO-5792.
Closes TURBO-5791.
