Identify Redcode correctly on its ACP endpoint

[filipeforattini]

What to build

Make the native ACP endpoint present Redcode rather than inherited OpenCode product identity. Its authentication guidance, agent metadata, session lifecycle, cancellation, tools, permissions, filesystem and terminal capabilities must describe the shipped Redcode binary while remaining compatible with ACP clients.

Acceptance criteria

• A black-box ACP test launches redcode acp and observes Redcode identity and Redcode authentication guidance.
• The endpoint passes the common initialize, session, prompt, streaming, tool, permission and cancellation baseline.
• Existing OpenCode-compatible configuration remains readable only where it is a deliberate compatibility input, never as public Redcode identity.