ci: skip heavy validation for documentation-only pull requests

[jackeyfaker77]

Summary

Split the core CI workflow into a lightweight planning job, an optional heavy validation job, and an always-reporting required test job.

Ordinary documentation-only changes, such as README and docs/ page updates, now skip dependency installation, build, and test work. Documentation or repository-authority files with dedicated contracts, including LICENSE, NOTICE, and the Astryx surface inventory, continue to select the relevant heavy validation.

The required test status still reports successfully when heavy validation is intentionally skipped and propagates planning or heavy validation failures.

Fixes #3998

Verification

• Added regression coverage for ordinary documentation-only, mixed documentation and code, and dedicated-contract changes.
• Verified that ordinary documentation-only changes produce heavy=false.
• Verified that code and dedicated-contract changes continue to select heavy validation.
• Ran the CI planner and workflow policy tests.
• Ran lint, format, build, typecheck, YAML parsing, and affected repository checks locally.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No