[Change] Output Snapshot Stability

[djm81]

Why

Users who script SpecFact in CI/CD pipelines rely on stable output: help text format, error message wording, JSON/YAML structured output shapes. Today, a refactoring PR can silently change help text or error phrasing with no test catching it. Snapshot testing makes output changes explicit, reviewable, and intentional — treating help text and structured output as versioned contracts.

What Changes

• NEW: syrupy dev dependency for pytest snapshot testing
• NEW: Snapshot tests for all command --help outputs
• NEW: Snapshot tests for structured output shapes (JSON/YAML) and key error messages
• EXTEND: CI pipeline to reject unreviewed snapshot changes
• EXTEND: pyproject.toml with syrupy configuration and hatch scripts

Acceptance Criteria

• Syrupy integrated and configured
• Snapshot tests exist for all command group help text
• Snapshot tests exist for key structured outputs and error messages
• hatch run snapshot-update and hatch run snapshot-check scripts work
• Documentation describes snapshot update workflow

OpenSpec Change Proposal: cli-val-02-output-snapshot-stability