harness: add Crush CLI support

[willwashburn]

Summary

Add Crush CLI as a Relay-supported coding harness.

Context

Crush is an open-source terminal coding agent from Charmbracelet. It should be evaluated as a lightweight local CLI harness target.

Sources:

• https://github.com/charmbracelet/crush

Scope

• Add Crush CLI metadata to the registry if feasible.
• Identify non-interactive/task flags and completion behavior.
• Investigate config/MCP support for Relaycast messaging.
• Add process command builder tests, smoke workflow, and docs.

Acceptance

• Crush has a documented Relay integration path or a clear no-go rationale.
• Supported modes run through Relay workflows with reliable output capture.
• Docs include install, auth/provider setup, transport, and limitations.