Approval-gate provider abstraction (platform-delegated or Fallout-server)

[ChrisonSimtian]

Part of #332.

A human "approve deploy" gate cannot live inside a build process — it's the one irreducibly-external piece. Model it as a pluggable provider:

• Delegated — GitHub Environments required-reviewers, Octopus manual-intervention, Azure DevOps approvals.
• Standalone — a Fallout-server approval when self-hosted (ties to the agent/runner [RFC] CD deployment agent/runner — lightweight, forge-agnostic, idle-and-wait #113).

RFC the gate contract (how a target/environment declares it needs approval, how the provider satisfies it, timeout/abort semantics).