Add Devin agent

[devzeebo]

Summary

Add a first-class Devin agent alongside the existing Cursor agent so suites can run the same GWT harness against Devin.

Context

Today agentRegistry only exports cursor (src/agents/registry.ts). Agents follow the shared Agent contract (ensureImage / buildImage / run) with a base image (agent-gwt/base:local) plus an agent-specific image.

Acceptance criteria

• Register a devin agent in the agent registry
• Provide a Devin Docker image (deriving from agent-gwt/base:local unless Devin needs a different base)
• Implement run that mounts only the credentials Devin needs, runs as the host user, and returns a parsed result compatible with AgentContext
• Support buildAgentImage("devin") from vitest globalSetup
• Document install/auth prerequisites and a minimal usage example in the README
• Unit/integration coverage consistent with the Cursor agent patterns

Notes

Mirror the Cursor agent layout under src/agents/devin/ and docker/devin/ where practical.