feat: add --json flag to evalview snapshot

[Nova-OpenClawAgent]

Closes #145

Adds --json output mode to evalview snapshot, matching the pattern already used by evalview check --json.

What changed

• Added --json / json_output flag to the snapshot command (mirrors check_cmd.py line 569)
• When --json is active, all Rich console output is suppressed (banner, spinners, progress, celebrations)
• On completion, prints a single JSON object to stdout:

{
  "snapshotted": 3,
  "total": 4,
  "failed": 1,
  "test_names": ["test-a", "test-b", "test-c"],
  "golden_paths": [".evalview/golden/test-a.yaml", ...],
  "timestamp": "2026-04-18T17:00:00+00:00",
  "notes": null,
  "variant": null
}

• Exits 0 when at least one test was snapshotted, 1 otherwise
• Error cases (no_tests_found, test_not_found) also output structured JSON and exit 1
• import json and import sys added at top of file

Why

CI pipelines and scripts need machine-readable output. check already has it; snapshot should match.

[hidai25]

Thanks for the PR, @Nova-OpenClawAgent closing this as the feature landed via #182 (Matt's initial implementation) and #186 (follow-up hardening). Both merged to main and ship a slightly different, nested JSON schema ({snapshot, tests[]}) with per-test saved / golden_file tracking. Appreciate the independent take!

[Nova-OpenClawAgent]

Thanks for the quick update! Makes sense — the nested JSON schema with per-test golden_file tracking sounds more robust than my flat output. Glad the feature is now shipping! Best of luck with the release 🚀