Add ACP adapter

[tanvesh01]

Closes #123

Open for testing!

Summary

• Add a standalone ACP adapter under source/acp/ that implements MVP Agent Client Protocol methods (initialize, authenticate, session/new, session/prompt, session/cancel) plus streaming updates and permission requests.
• Add session-level model selection via session/set_config_option and return model config options in session/new.
• Improve tool-call UX for ACP clients by including richer tool titles (notably shell command previews), and document dev/prod Zed setup examples.

Validation

• npm run format
• npm run build
• npm run test:run

Octo's ACP in Zed