feat: add severity levels and suppressions to the layer validator #21

Description

Every validator finding is an error today: the hook exits 2 and blocks Claude. Several planned checks are judgment calls that should be reported without blocking, for example a presentation package that does not depend on its own feature's domain, or a repository interface that returns another feature's model.

Add severity levels and a way to suppress a finding with a stated reason, so the stricter checks can land without making the hook noisy.

Requirements

• Each rule has a severity: error or warning.
• Errors keep the current behavior: exit 2, and the hook blocks the edit.
• Warnings are printed but the exit code stays 0 when there are no errors.
• A finding can be suppressed with a comment that requires a reason, for example # ffca-ignore: <rule-id> — <reason> in the pubspec.yaml or // ffca-ignore: <rule-id> — <reason> in Dart. A suppression without a reason is itself an error. The pubspec form is in feat(validator): add severity levels and suppressions #30; the Dart form lands with feat: check imports and barrels in the layer validator #25, the first check that reads Dart files.
• Each rule has a stable ID used in output and suppressions.
• --all prints how many findings are suppressed.
• Tests and fixtures cover both severities and suppression.
• The Hooks section of README.md and the ## Hooks section of CLAUDE.md describe the new behavior.
• All CI/CD checks are passing.
• There is no drop in the test coverage percentage.

Additional Context

Prerequisite for the warning-level checks in the feature ownership, import and barrel, and type-level issues.