feat(ci): prefer hosted runners, fallback to self-hosted via reusable workflows

[polyipseity]

Summary

• Add reusable workflows for release, docker, and checks; callers now use hosted-first → self-hosted fallback.
• New files: .github/workflows/{release,docker,check}-reusable.yml.
• Callers updated: .github/workflows/{release,docker,check}.yml call reusable workflows for try/fallback.
• Fallback gating uses if: ${{ failure() }} so fallback runs only when the try job fails.
• continue_on_error is passed into reusable workflows as an input and applied at job level.
• Clarifying comments added about PR UI duplication and cosmetic failures when fallback runs.
• Fallback runs_on set to self-hosted.

Notes

• Branch: feat/prefer-hosted-fallback-reusable
• Expect PR UI to show duplicated/expanded checks when fallback executes; this is cosmetic (documented in comments).