[ACT-07] Add a safe handoff fixture without a human adapter

[EmersonBraun]

Goal

Create a handoff context with an invalid edit root or no human adapter. The agent should produce a safe handoff record that stops short of creating a public guide.

Evidence

• Human guide: docs/agents/ecosystem-doc-bridge-handoff-author.md
• Agent test: registry/ecosystem-doc-bridge-handoff-author/agent.test.ts

Acceptance criteria

• The missing safety condition is explicit.
• Edit roots and checks are validated.
• The output requires human review.
• The maintainer resolution is captured in the fixture rationale.

Human gate

A Doc Bridge maintainer must approve the safe resolution.