docs: record active validation merge checks

[codeforester]

Summary

Record that the fail-closed validation aggregate contexts are now active
default-branch merge checks.

Issue

Refs #540

Changes

• update CONTRIBUTING.md step 7 with the active merge-check contexts;
• change docs/ci-policy.md from planned to active policy wording;
• retain the repeatable effective-ruleset and classic-protection readback.

Validation

• git diff --check
• ./tests/ci-policy-contract.sh
• live ruleset readback confirms Product validation and Quality contract
  with Actions integration ID 15368.

[codeforester]

Closing this PR because issue #540 is categorized as ci, and the repository branch policy requires the PR branch category to match that issue category. The same documentation change will be recreated from a canonical ci/540/... branch.