Add structured validate output with text/json/yaml formats

[MicheleBonus]

Motivation

• Provide machine-readable output for the validate command so callers can consume structured results as JSON or YAML in addition to existing human-readable text.
• Preserve existing text UX and current exit-code semantics (including --strict) while enabling automated tooling to inspect validation outcomes.

Description

• Add --format (text|json|yaml, default text) to the validate subparser via validate_parser.add_argument.
• Refactor _validate_command to build a structured result dict containing an overall status, per-file entries with file, status, warnings, and errors, plus top-level warnings and errors arrays.
• Keep the existing colored text rendering when --format text is used, and emit json.dumps(result, indent=2) for json and yaml.safe_dump(result) for yaml (with a PyYAML check).
• Preserve existing exit-code logic so --strict still causes failures on warnings and errors always return non-zero.
• Add tests/test_cli_validate.py to assert JSON output shape and strict-mode behavior.

Testing

• Ran pytest -q tests/test_cli_validate.py tests/test_cli_protocol_output.py tests/test_cli_plan.py and all tests passed.
• Test summary: 7 passed (covers new JSON-shape assertions and strict vs non-strict exit behavior).

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Codex Task