[codex] Add ballin doctor health check

[JBallin]

Summary

Closes #187.

Adds a small ballin doctor surface backed by the shared setup readiness helper from #173 / PR #189. The command stays focused on Ballin-managed development environment health and does not expand into broader CLI routing.

What changed

• Adds ballin doctor handling to the existing ballin command.
• Formats readiness helper checks as concise OK, WARN, ERROR, and INFO lines.
• Treats warning-only doctor output as exit 0, required check failures as exit 1, and invalid ballin doctor usage as exit 2.
• Documents the new command and explicitly keeps brew doctor out of this slice.
• Adds isolated tests with temp config, temp PATH, and command stubs.

Scope notes

• Surfaces current helper checks for Node.js runtime, command shims on PATH, config readability, Gist host, Gist ID, gh availability, and gh auth status.
• Leaves raw helper metadata helper-only.
• Leaves install/update readiness output to Consider using setup readiness in install and update output #191.
• Leaves Homebrew-specific brew doctor readiness to Consider Homebrew readiness check via brew doctor #188.
• Leaves broader ballin <subcommand> routing to Evaluate CLI command naming overhaul #194.
• Leaves reliable ballin_config reset recovery and future doctor copy polish to Make ballin_config reset recover missing or malformed config #199.

Validation

• npm test (226 passing)
• git diff --check