Document native ACP setup and expose terminal authentication

[igorcosta]

What changed

• Add a complete native ACP guide for Zed, JetBrains IDEs, JetBrains Air, GitHub Copilot, and future ACP-compatible ADEs.
• Advertise Autohand setup as an ACP terminal authentication method during initialization.
• Correct the ACP login guidance to use autohand --login.
• Explain why manually configured Air agents use a generic icon and how ACP Registry metadata supplies branded icons.
• Add contract tests for the guide and ACP handshake.

Why

Autohand Code already supports native ACP through autohand --acp, but users had no maintained setup guide. The ACP Registry also validates that agents expose an authentication path, which the native adapter did not previously advertise. Manual Air entries therefore worked but appeared as generic custom agents rather than branded registry agents.

Impact

Users can configure Autohand Code consistently across supported ACP clients. Registry clients can discover the interactive setup flow, and the repository now documents the distinction between manual ACP configuration and branded Registry installation.

Validation

• Focused ACP and documentation tests: 68 passed
• Lint: passed
• Full proof unit gate: 471 test files passed, 2 skipped; 7,109 tests passed, 26 skipped
• Production ESM, CJS, and type declaration build: passed
• Tuistory built-terminal gate: 3 files and 33 tests passed