fix: Add back temporary workflow to publish main to ghcr

[brendan-kellam]

This PR adds back a workflow to publish main to ghcr since the previous PR removed that functionality.

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Note

Introduces reusable CI for building and publishing multi-architecture Docker images and wires them into dev and prod releases.

• New reusable workflows: ._github/workflows/_build.yml (per-platform build, sign, and digest artifact upload) and ._github/workflows/_merge.yml (merge digests into multi-platform manifest and push)
• New release-dev.yml: builds from main and publishes ghcr.io/sourcebot-dev/sourcebot:main
• Refactors release-prod.yml: replaces inline build/manifest steps with calls to _build.yml and _merge.yml, adds publish-to-registry job post-tagging, and updates job dependencies/cleanup; preserves tagging (vX.Y.Z, latest) and GitHub Release creation

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