Deployment dependencies: core evaluation engine

[adityachoudhari26]

Sub-issue of #1067.

Implement the workspace-engine logic that gates a release target's desired release on its declared deployment dependencies.

For a release target (A, env, R), for each row in deployment_dependency where deployment_id = A:

• Look up the dependency B's current successful release on resource R (any environment, picks one).
• Evaluate the row's version_selector (CEL) against that version.
• All edges must pass; otherwise no desired release is computed.

Lives in the desired-release pipeline as a non-policy-rule evaluator alongside the existing policy evaluators.