chore: remove retired Sandboxwich publication authority

[haasonsaas]

Summary

• make the standalone container workflow pull-request validation only
• remove retired release workflow stubs and all remaining GHCR login, index, signing, artifact-upload, and promotion logic
• mark this repository as a historical read-only snapshot whose source and publication authority live in evalops/mono
• route new issues to the canonical Mono repository and enforce the authority boundary in repository tests

Authority boundary

The canonical Sandboxwich source is evalops/mono/platform/execution/sandboxwich. This repository must not publish artifacts or act as a promotion/source authority. This change does not archive, delete, or lock the repository and does not alter repository or package permissions.

Verification

• python3 scripts/test-repository-rules.py (18 passed)
• python3 scripts/test-release-readiness.py (9 passed)
• parsed every .github/**/*.yml with PyYAML
• git diff --check 65ba71b639b9fd6d37109897de2488e8d6e30ecc..c54d74dba16882f1d6fc4f8bc4409fe690174ee6
• cargo fmt --check (passed)
• cargo clippy --workspace --all-targets -- -D warnings (passed)
• cargo test --workspace (passed at c54d74dba16882f1d6fc4f8bc4409fe690174ee6 in the clean publication checkout)

An independent source-orb run at parent commit b9966b1c54f771e6af6526c3b028062f0b1c1b84 had one unrelated worker-test failure after the broader suites passed: provider::tests::agent_sandbox_launch_fails_closed_without_setsid expected exit marker 127 but observed an empty string (290 worker tests passed, 1 failed). This branch does not touch that source or test; protected CI should classify the current exact head independently.

Postgres-backed tests were not enabled locally because SANDBOXWICH_TEST_POSTGRES_URL was not provided.