Support multi-role transfer proof bundles for source/target/sandbox runs

[chubes4]

Goal

Expose a reusable WP Codebox proof-bundle shape for transfer rigs that involve source, target, and sandbox runtimes instead of one isolated runtime.

Scope

Transfer rigs need to connect evidence across multiple disposable WordPress runtimes:

• source inventory/runtime evidence
• target before/after evidence
• sandbox rehearsal evidence
• preview/session evidence
• probe diagnostics
• digest-linked artifact refs

Acceptance criteria

• A transfer proof bundle can reference multiple runtime sessions/roles safely.
• Existing single-runtime artifact verification still works.
• Reviewer-facing evidence rejects localhost URLs, local paths, private URLs, and secret-shaped values.
• Conductor/Homeboy Extensions can consume the bundle without repo-specific shims.

Parent context

• Conductor Design: Sandbox Runtime contract #1 / Add agent sandbox run and WordPress ability #21