Adopt central Ruff lint and format workflow #4

Evidence

CI currently installs unpinned Ruff and runs ruff check . plus ruff format --check .; pyproject.toml contains the repository-specific lint rules. The existing CI matrix and pytest/coverage checks are separate and should remain intact.

Scope

Replace the local Ruff install/check job with a thin caller of the future central Ruff reusable workflow. Keep the pyproject policy and choose one source of tool-version truth. Document developer lint setup.

Acceptance criteria

• Call centrally maintained Ruff check/format workflow pinned to a full immutable commit SHA and exact Ruff/Python versions.
• Preserve ruff check . and ruff format --check . semantics; no autofix.
• Preserve existing syntax, test, platform, and coverage jobs.
• Keep permissions read-only and pass CI.

Blocked on DevOpsDerek/workflows#5; leave unassigned in Project #2 Backlog until implementation begins.