docs: add output and exit-code contract

[CoderDeltaLAN]

Summary

• Expands docs/OUTPUTS.md into an initial CLI output contract and examples document.
• Adds docs/EXIT-CODES.md for the current check and init exit-code behavior.
• Links the new exit-code contract from README.
• Records the documentation addition in CHANGELOG under Unreleased.

Scope

• Documentation only.
• No code changes.
• No CI changes.
• No dependency changes.
• No release, tag, or PyPI changes.

Validation

• Pre-stage checks passed.
• Staged review passed.
• Pre-push strong check passed.
• 118 unit tests passed locally with dev dependencies.
• Ruff passed.
• Text hygiene passed.
• Git whitespace checks passed.
• CLI smoke confirmed expected exit codes for --version, check JSON, no-result check, and init without mode.

Risk

• Low. Documentation-only contract clarification.
• Main risk is over-promising stability; mitigated by explicit pre-v1.0 and non-security-guarantee wording.

Rollback

• Revert this documentation-only commit.