refactor: Port Cargo task and contract knowledge

[anthonyshew]

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.

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