perf(ci): path-filtered builds — skip //cdk:test on non-CDK PRs — follow-up to #363

[scottschreckengaust]

Parent: #363. Spun from #363's DoD by the orchestrator; needs maintainer approved (ADR-003).

Goal

Make the expensive //cdk:test step conditional (e.g. dorny/paths-filter) so docs/CLI/agent-only PRs skip it while the required build check still reports success.

Scope

• Path filter drives whether cdk test runs.
• Annotate what was skipped so a skipped suite is never mistaken for a covered one (silent-truncation guard).
• Biggest win for the non-CDK long tail.

Refs #363