ci(windows): make PR checks opt-in and main jobs advisory

[pimlock]

Summary

Windows CI currently runs expensive lint/tests on every PR mirror and merge group. Make PR validation opt-in with test:windows, remove merge-group runs, and keep main/manual validation advisory so Windows failures do not fail those workflows.

The sample x64 PR job took 24m 53s: 9m 49s linting, 9m 14s compiling/running tests, and 4m 52s cleaning/saving the cache. Test execution itself took only 70 seconds. The target-cache save failed with an explicit GitHub rate-limit warning after cleanup and compression. sccache recorded 213 hits, 792 misses, and 792 write errors, so caching is only partly effective. Rate limiting may also explain the sccache write failures, but the statistics alone do not establish that cause.

These timings and cache failures motivate reducing automatic Windows runs while cache reliability and warm-run performance are investigated. This PR changes when validation runs and how failures affect workflow status; it does not fix the cache failures.

Related Issue

No issue required: localized CI configuration maintenance requested by a maintainer; no product or runtime behavior changes.

Changes

• Require test:windows through the existing PR gate for PR lint/tests. Created the repository label; applying it requires re-running the current mirror workflow or pushing another mirrored commit.
• Remove the merge-group trigger.
• Set job-level continue-on-error: true on main/manual cache-seed and binary-build jobs. Opt-in PR failures remain visible normally.
• Update CI documentation, architecture guidance, and the Windows contributor skill.

Testing

• mise run pre-commit passes
• actionlint passes for .github/workflows/windows-msvc.yml
• git diff --check passes
• Unit/E2E additions are not applicable to this workflow-only change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated