fix: guide unconfigured engine auth

[CompleteDotTech]

Summary

• fail before forwarding coven auth login to an Anthropic OAuth flow when no Coven Code OAuth client ID is configured
• point the doctor credentials hint at API-key, Claude Code, and configured-OAuth paths
• document the engine auth boundary and supported alternatives

Root cause

coven auth login blindly delegated to Coven Code. Its Anthropic OAuth login requires COVEN_CODE_ANTHROPIC_OAUTH_CLIENT_ID, so a normal installation reached a browser-login path that could not succeed.

User impact

Users now receive an actionable local error before the engine starts the unusable OAuth flow. Existing deployments with a registered client ID continue to delegate to the engine unchanged.

Validation

• cargo fmt --check
• git diff --check
• targeted current-tree secret scan
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --locked (1,219 passed; 1 ignored)
• built CLI runtime checks: unconfigured coven auth login fails before engine launch with the supported alternatives; coven auth status and configured-client coven auth login pass through to normal engine resolution
• GitHub CI passed on Ubuntu, Windows, macOS, Linux x64, and Windows npm onboarding
• the repository-wide historical secret scan was started, but stopped after two minutes with no findings/output because it scans every historical blob serially; CI passed the full secret gate

[CompleteDotTech]

Ready for review, @BunsDev.

This fixes the unconfigured coven auth login dead end, preserves coven auth login --help, and clarifies the related doctor guidance. The exact head is 830ca94; all CI checks are green, including Rust on Ubuntu and Windows plus npm onboarding on macOS, Linux, and Windows.