harness: add Mistral Vibe CLI support

[willwashburn]

Summary

Add Mistral Vibe CLI as a Relay-supported coding harness.

Context

Mistral Vibe is a terminal-native coding agent with local/offline-oriented usage. It is a candidate for process or PTY-backed Relay support.

Sources:

• https://docs.mistral.ai/mistral-vibe/overview
• https://docs.mistral.ai/mistral-vibe/cli-reference

Scope

• Add Mistral Vibe CLI metadata to the registry if feasible.
• Identify non-interactive/headless flags and completion behavior.
• Investigate Relaycast messaging options via config, MCP, or prompt injection.
• Add tests and docs.

Acceptance

• Vibe has a documented Relay integration path or a clear no-go rationale.
• Supported modes include command-builder and output completion coverage.
• Docs include install, auth/model setup, transport, and limitations.