Add CI rule-linting workflow

[jpl1337]

Split out of #241 per review — this is the GitHub Actions half: the new linting workflow, plus the v2.yml cleanup requested in that PR's open questions.

Changes

1. .github/workflows/lint-rules.yml — runs on PRs touching rules/, src/ or the cargo manifests (and on push to master). It builds chainsaw from the PR's own source rather than downloading a release, so a rule that depends on a schema change in the same PR is validated against that change. Uses Swatinem/rust-cache, so only the first run pays the full build. Depends on #242 (or another lint fix) landing first for the check to actually gate on rule failures — until then it builds and lints but chainsaw lint still exits 0 on failure.

2. v2.yml — replaced the archived actions-rs/toolchain@v1 with dtolnay/rust-toolchain@stable across all 5 jobs, per the open question in #241. Behavior is equivalent: profile: minimal/override: true/default: true are no-ops for dtolnay/rust-toolchain (it always installs minimal and sets itself as the active toolchain), and target/targets map directly. v1.yml is untouched — out of scope per the original ask.

Verification

• yamllint on both files: no new warnings/errors vs. current master (pre-existing line-length debt in v2.yml run: lines is unchanged, not introduced here).