refactor: Remove runtime toolchain dispatch (#13584)
## Intent
Remove the final runtime consumers of the behavioral toolchain registry
while keeping discovery deletion as a separate reviewable step.
**Base:** `main`.
## Changes
- Project compile-cache environment decoration from immutable per-scope
task contracts.
- Move Cargo post-write prune finalization into generation-owned prune
knowledge.
- Derive task-entrypoint provenance from active candidates instead of
registered toolchains.
- Stop retaining `ToolchainRegistry` on completed package graphs.
- Update architecture documentation for construction-scoped producers
and knowledge-backed runtime consumers.
- Preserve command-override, sccache composition, Docker
synchronization, and nonfatal Cargo finalization behavior.
## Testing
- `cargo test -p turborepo-repository cargo::test` (41 passed)
- `USER=vercel cargo test -p turborepo-task-executor` (18 passed)
- `cargo test -p turbo --test cargo_workspace_test` (57 passed)
- `cargo clippy -p turborepo-repository -p turborepo-task-executor -p
turborepo-lib --all-targets -- -D warnings`
- `cargo fmt --check`
Advances TURBO-5798.
