fix(ci): skip platform matrix for docs-only PRs

[sanohiro]

Summary

• classify true docs-only pull requests with the shared fail-closed pr-tier.sh predicate loaded from protected main
• skip the Linux/macOS compiler matrix only for that exact tier while retaining full checks for compiled prose, deletions, unknown paths, stacked PRs, pushes, and manual runs
• add Platform CI (required) as the stable aggregate for branch protection
• make local docs-only preflight and CI consume the same predicate

Trust boundary

• both database and platform scope classifiers fail closed for non-main PR bases
• both load classifier machinery only from refs/remotes/origin/main
• the first PR containing the new predicate runs the full matrix because the trusted base cannot yet prove the lighter tier

Verification

• bash scripts/test-pr-workflow.sh
• bash scripts/test-db-ci-scope.sh
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml")'
• findings-fixed scripts/pre-pr.sh on e8e91c5f

After this PR merges, branch protection will replace the three platform-specific required contexts with Platform CI (required). The platform jobs still run on code PRs; true docs-only PRs receive the aggregate success without launching them.