CLI: add stable JSON output for validate and audit

[eun7661010]

Problem

validate and audit currently print readable terminal messages. CI jobs and coding agents have to interpret those lines instead of consuming a stable result object.

Proposed change

Add an opt-in --format json flag to both commands. Keep the current human-readable output as the default.

The JSON result should include:

• a version string for the output contract;
• the command name and package version;
• the overall ok value and process exit code;
• validation issues or audit checks as structured arrays;
• file names only, never absolute paths.

Write the JSON document to stdout and operational errors to stderr so callers can redirect it safely.

Acceptance criteria

• exam-hwpx validate INPUT --format json emits one valid JSON document.
• exam-hwpx audit INPUT OUTPUT --format json does the same for successful and failed audits.
• Existing commands and exit codes remain backward compatible.
• Tests cover success, validation failure, audit failure, and a non-UTF-8 Windows console.
• README and llms.txt include one short example.
• Output contains no absolute path, even when the caller supplies one.

Useful starting points: src/exam_hwpx_kit/cli.py, src/exam_hwpx_kit/validation.py, src/exam_hwpx_kit/audit.py, and tests/test_cli.py.

Please use only the bundled synthetic fixture. Do not attach real exams, private templates, credentials, or personal file paths.