feat: Add Factory Droid as built-in agent

[ironerumi]

Factory Droid supports ACP natively via droid exec --output-format acp (announced Jan 2026).

This adds droid as a built-in agent using the native CLI command, requiring the Droid CLI to be installed.

Droid ACP details:

• Command: droid exec --output-format acp
• Protocol: Standard ACP JSON-RPC 2.0 over stdio
• Auth: FACTORY_API_KEY or droid interactive login
• Docs: https://docs.factory.ai

Tested: macOS arm64, acpx 0.1.0 (local build), Droid CLI latest.