build(ci): Enable hosted Phase 0 lanes

[sperlingxx]

Summary

• Restore path-selected Breeze CPU validation for HPA pull requests targeting dev.
• Restore path-selected documentation validation on a GitHub-hosted runner.
• Preserve current dev action pins and add focused workflow-contract tests.

Boundary

This successor carries only the hosted Breeze/documentation scope from draft PR #25:

• .github/workflows/breeze.yml
• .github/workflows/docs.yml
• .github/scripts/test_phase0_hosted_workflows.py

The HPA fork receives only the CPU Breeze lane; the unavailable GPU lane remains upstream-only. Documentation publishing remains guarded to upstream push events. The jobs use read-only permissions, disabled checkout credential persistence, and five-minute job timeouts.

It does not change the Phase 0 aggregate, Blossom/Jenkins, dependency-graph behavior, Velox source/build code, or the trigger-pruning workflows owned by #43.

Related: #27
Supersedes the Breeze/documentation portion of #25.
Trigger-pruning replacement: #43.

Validation

• 6 focused hosted-workflow contract tests passed.
• All applicable repository pre-commit hooks passed, including Ruff, YAML formatting/lint, zizmor, and GitHub Actions validation.
• git diff --check passed.
• Commit is based directly on current dev at c1234711169ddef2bf5a6ae370f0f6935c0dbe39.

Promotion state

Draft pending GitHub checks and fresh hosted canaries on this final SHA. No native or GPU coverage is claimed.