feat(config): add validated config view command

Summary

• add juice config view <configurable-import-path> to print the fully merged active configuration as TOML
• include merged source file paths before the rendered configuration
• surface validation/load failures with a clear CLI error

Closes #292 (closed).

Validation

• WSL Ubuntu: /mnt/c/Users/sebas/Documents/BountyBar/.tools/juice-wsl-venv/bin/python -m pytest tests/juice/cli/test_config.py -q -> 6 passed in 45.16s

Note: the same focused test cannot collect under native Windows because upstream imports socketserver.UnixStreamServer, which is only available on Unix-like platforms.

AI disclosure

I used Codex/LLM assistance to draft and review this change, then ran focused local verification before submission.