Remove repo-local _* Rust tool wrappers and call soldr directly

[zackees]

Problem

#53 replaced the old Rust toolchain resolution logic with soldr, but the repo still exposes repo-local _cargo, _rustc, and _rustfmt scripts. Those wrappers now mostly add another indirection layer around soldr:

./_cargo   -> uv run soldr cargo "$@"
./_rustc   -> uv run soldr rustc "$@"
./_rustfmt -> uv run soldr rustfmt "$@"

That means developers and agents still have to remember fbuild-specific _ commands even though soldr already solves the underlying problem: selecting the correct rustup-managed toolchain from rust-toolchain.toml via rustup which.

The desired steady state is simpler: fbuild should not require or recommend _cargo, _rustc, _rustfmt, or other repo-specific Rust build tool shims. Use soldr ... directly, with uv run soldr ... where the dev-tool virtualenv is needed.

Proposed fix

1. Update docs and agent guidance to prefer direct soldr commands:

   • uv run soldr cargo ...
   • uv run soldr rustc ...
   • uv run soldr rustfmt ...
   • uv run soldr clippy-driver ...

2. Remove _cargo, _rustc, and _rustfmt if no CI/job/script still depends on them.

3. Update any scripts, docs, hooks, or examples that mention _cargo, _rustc, or _rustfmt.

4. Keep soldr responsible for maintaining the correct Rust tool version by resolving through rustup and the checked-in rust-toolchain.toml.

Acceptance criteria

• rg "_cargo|_rustc|_rustfmt" has no remaining user-facing recommendations, except historical references if explicitly needed.
• _cargo, _rustc, and _rustfmt are removed, or any remaining wrapper has a documented compatibility reason and is no longer recommended.
• CI and local docs use uv run soldr ... / soldr ... directly.
• ci/hooks/tool_guard.py continues to allow soldr ... and does not require repo-local _ tools.
• uv run soldr cargo check --workspace --all-targets works as the canonical Rust check command.

Related

• Replace Rust toolchain trampolines with soldr #53: initial migration from hand-rolled Rust toolchain trampolines to soldr. This follow-up removes the leftover repo-local _* command surface instead of keeping it as the recommended workflow.

[zackees]

Additional requirement: update the agent/tool hooks as part of this issue.

Today ci/hooks/tool_guard.py allows Rust commands through uv run ..., soldr ..., and the repo-local _cargo / _rustc / _rustfmt compatibility wrappers. Once this cleanup lands, the hooks should require direct soldr usage for Rust tooling and stop treating the _ wrappers as an accepted path.

Expected hook behavior:

• Allow soldr cargo ..., soldr rustc ..., soldr rustfmt ..., soldr clippy-driver ....
• Allow uv run soldr cargo ... and equivalent uv run soldr <tool> ... forms when running through the dev-tool environment.
• Reject bare cargo, rustc, rustfmt, and clippy-driver as today.
• Reject or no longer document _cargo, _rustc, and _rustfmt after those files are removed.
• Update ci/hooks/test_tool_guard.py and hook docs so the enforced workflow matches the new canonical commands.