feat: replace OpenClaw enrollment form with room command

[atlas-maxjb]

Summary

• replace the deployed OpenClaw enrollment-code and optional SSH form with Desktop room selection
• generate a signed, short-lived copyable openclaw buzz enroll --code command from the bundled provider
• preserve the generic provider protocol and add focused room-picker coverage

Validation

• cargo fmt --all -- --check
• git diff --check
• Full Rust/linking and desktop build validation runs in the fork macOS DMG workflow because the local container lacks the native C linker and desktop dependencies.