feat(cli): add --config option to generate

[omer-cengel]

Summary

sqlcj generate can now load a configuration file named by --config <path>, so generation can run from any working directory. Without the option, sqlcj.yaml in the working directory remains the default and behavior is unchanged.

Changes

• Add --config <path> to generate, defaulting to sqlcj.yaml. A relative value is resolved against the working directory; relative schema, queries, and java.out paths inside the file still resolve against the file's own directory.
• A missing or invalid explicit file reports the existing one-line sqlcj: configuration diagnostic and exits with status 1, with no fallback to sqlcj.yaml.
• Add CLI subprocess tests that generate from a sibling directory without writing to it, and that reject a missing explicit file without generating output.
• Document the option, its default, and path resolution in the README, configuration reference, and quickstart.

Scope and non-goals

• No short option alias, environment variable, or parent-directory configuration discovery.
• Configuration path resolution inside the file, exit codes, and diagnostic wording are unchanged.