harness: add Cline CLI support

[willwashburn]

Summary

Add Cline CLI as a Relay-supported coding harness.

Context

Cline now has a documented CLI/headless surface in addition to the editor extension. Relay should evaluate it as a spawnable coding harness.

Sources:

• https://docs.cline.bot/cline-cli/overview
• https://docs.cline.bot/cline-cli/headless

Scope

• Add Cline CLI metadata to the registry and generated outputs.
• Implement non-interactive/headless command builder support where available.
• Investigate MCP/config hooks for Relaycast messaging.
• Add readiness/completion tests and docs.
• Document overlap or differences from Kilo/Roo-style harnesses.

Acceptance

• Cline CLI can be run through Relay workflows or is explicitly ruled out with rationale.
• Supported modes have tests for command construction and output completion.
• Docs include install, auth/provider setup, transport, and limitations.