ci: split fast PR Rust CI from full post-merge Cargo CI

[bolinfest]

Summary

Split the old all-in-one rust-ci.yml into:

• a PR-time Cargo workflow in rust-ci.yml
• a full post-merge Cargo workflow in rust-ci-full.yml

This keeps the PR path focused on fast Cargo-native hygiene plus the Bazel build / test / clippy coverage in bazel.yml, while moving the heavyweight Cargo-native matrix to main.

Why

bazel.yml is now the main Rust verification workflow for pull requests. It already covers the Bazel build, test, and clippy signal we care about pre-merge, and it also runs on pushes to main to re-verify the merged tree and help keep the BuildBuddy caches warm.

What was still missing was a clean split for the Cargo-native checks that Bazel does not replace yet. The old rust-ci.yml mixed together:

• fast hygiene checks such as cargo fmt --check and cargo shear
• argument-comment-lint
• the full Cargo clippy / nextest / release-build matrix

That made every PR pay for the full Cargo matrix even though most of that coverage is better treated as post-merge verification. The goal of this change is to leave PRs with the checks we still want before merge, while moving the heavier Cargo-native matrix off the review path.

What Changed

• Renamed the old heavyweight workflow to rust-ci-full.yml and limited it to push on main plus workflow_dispatch.
• Added a new PR-only rust-ci.yml that runs:
  • changed-path detection
  • cargo fmt --check
  • cargo shear
  • argument-comment-lint on Linux, macOS, and Windows
  • tools/argument-comment-lint package tests when the lint itself or its workflow wiring changes
• Kept the PR workflow's gatherer as the single required Cargo-native status so branch protection can stay simple.
• Added .github/workflows/README.md to document the intended split between bazel.yml, rust-ci.yml, and rust-ci-full.yml.
• Preserved the recent Windows argument-comment-lint behavior from e02fd6e1d3 in rust-ci-full.yml, and mirrored cross-platform lint coverage into the PR workflow.

A few details are deliberate:

• The PR workflow still keeps the Linux lint lane on the default-targets-only invocation for now, while macOS and Windows use the broader released-linter path.
• This PR does not change bazel.yml; it changes the Cargo-native workflow around the existing Bazel PR path.

Testing

• Rebasing this change onto main after e02fd6e1d3
• ruby -e 'require "yaml"; %w[.github/workflows/rust-ci.yml .github/workflows/rust-ci-full.yml .github/workflows/bazel.yml].each { |f| YAML.load_file(f) }'