docs: CONTRIBUTING says clippy is not gated at PR time, but ci.yml runs -D warnings clippy on PRs

Read CONTRIBUTING.md around lines 46 and 50, then inspect the pull_request-triggered jobs in .github/workflows/ci.yml, especially the clippy job and its comment. Update the bullet and inline comment so they accurately describe the PR-time jobs and gates. Done means the CONTRIBUTING.md description matches the workflow's actual PR behavior.

Summary

CONTRIBUTING.md tells contributors that clippy is "NOT gated at PR time" and that PR CI runs "only the cheap gates", but ci.yml runs a -D warnings clippy job on every Rust-touching PR, plus several further PR-time gates the paragraph never mentions. A contributor who trusts CONTRIBUTING is surprised by CI.

Background

The paragraph describes a CI arrangement that predates the clippy job being re-added to ci.yml. The job's own comment even notes it re-adds what earlier issues removed, so the docs and the workflow now disagree.

Proposed Solution

Rewrite the bullet and the inline comment against the actual PR job list, which is trivially enumerable from ci.yml.

Implementation Notes

• CONTRIBUTING.md:46: "cargo clippy --workspace --all-targets ... -D warnings # NOT gated at PR time; yours to run".
• CONTRIBUTING.md:50: "PR-time CI runs only the cheap gates ... Clippy and the general unit suite are not enforced on your PR."
• .github/workflows/ci.yml:53-57 (confirmed): triggers on pull_request: branches: [main].
• .github/workflows/ci.yml clippy job runs cargo clippy -p mlxcel --lib --tests -- -D warnings (confirmed -- -D warnings at :214) when Rust files changed.
• Additional PR-time gates unmentioned by the paragraph: crate-versions, kernel-dtype-keys, llama-compat-manifest, mlx-pin, cross-repo-refs (advisory), xla-compile, xla-link, cuda-sm70-compile.

Acceptance Criteria

• CONTRIBUTING's description of PR-time CI matches the jobs ci.yml actually runs on PRs.

Original Suggestion

Title: docs: CONTRIBUTING says clippy is not gated at PR time, but ci.yml runs -D warnings clippy on PRs

CONTRIBUTING.md tells contributors that clippy is "NOT gated at PR time" and that PR CI runs "only the cheap gates" — but ci.yml runs a -D warnings clippy job on every Rust-touching PR, plus seven further PR-time gates the paragraph never mentions. A contributor who trusts CONTRIBUTING gets surprised by CI.

Evidence

• CONTRIBUTING.md:46 — "cargo clippy --workspace --all-targets … # NOT gated at PR time; yours to run"
• CONTRIBUTING.md:50 — "PR-time CI runs only the cheap gates: cargo fmt, cargo deny and a workspace crate-version consistency check … Clippy and the general unit suite are not enforced on your PR."
• .github/workflows/ci.yml:53-57 — runs on pull_request: branches: [main]
• .github/workflows/ci.yml:155-215 — the clippy job runs cargo clippy -p mlxcel --lib --tests -- -D warnings when Rust files changed; its own comment at :207-209 notes it re-adds what earlier issues removed

Additional PR-time ci.yml gates unmentioned by the paragraph: crate-versions (:216), kernel-dtype-keys (:239), llama-compat-manifest (:260), mlx-pin (:299), cross-repo-refs (:345, advisory), xla-compile (:417), xla-link (:576), cuda-sm70-compile (:657).

Suggested fix

Rewrite the bullet and the inline comment against the actual job list, which is trivially enumerable from ci.yml.

Acceptance criteria

• CONTRIBUTING's description of PR-time CI matches the jobs ci.yml actually runs on PRs