ci: remove automated release image publishing

[Che-Zhu]

Summary

• Remove the automated Release Image workflow.
• Keep PR CI focused on lint, test, and build.
• Leave runtime image workflow as manual-only.
• Document the manual GHCR image tagging policy in AGENTS.md.

Behavior

• GitHub Releases no longer trigger image builds or pushes.
• Image release is manual for now.
• Manual images use ghcr.io/fullagent/fulling.
• Formal releases use version + sha + latest tags.
• Prereleases use version + sha tags and must not update latest.
• Temporary validation images use purpose-prefixed SHA tags such as dev-<short-sha>.
• Remaining workflows are CI and manual Build Runtime Image.

Validation

• Remaining workflow YAML parses with Ruby YAML loader.