fix(release): decouple releases from staging availability

[FelixTJDietrich]

Summary

• run staging deployment as an independent workflow after a successful Docker build
• let standalone releases be resumed manually from an already-built commit SHA
• update GHCR cleanup to the token-format-compatible, multi-arch-safe v3.1.0 action
• document the independent failure and recovery paths

Root cause

The standalone release listened to the overall Build and Push Docker Image conclusion, but that workflow also contained staging deployment. A gateway outage therefore changed a successful image build into a failed workflow and suppressed the release. The cleanup workflow separately pinned an action version that predates the current GitHub installation-token format.

Validation

• actionlint on all modified workflows
• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test: 1,751 passed, 1 skipped
• docs markdown lint

No changeset: this changes CI and release orchestration only.