feat(check): add --json flag to check command

[DaleSeo]

Closes #663

Summary

Adds a --json flag to skills check that outputs structured JSON to stdout. This enables CI scripts and other tooling to programmatically detect stale skills without parsing ANSI-colored text. Follows the same pattern established by #558 (list --json).

Output format

$ npx skills check --json

[
  { "name": "my-skill-1", "source": "owner/repo" },
  { "name": "my-skill-2", "source": "owner/repo" }
]

Key properties:

• No ANSI escape codes in output
• Empty array [] when no updates available or no skills tracked
• Output is always a JSON array, consistent with skills list --json

Changes

File	Change
src/cli.ts	Parse --json flag, output JSON array, suppress human-readable output in JSON mode, add help text and example
src/cli.test.ts	3 new tests (valid JSON output, array structure, no ANSI codes)

Tests

All 12 CLI tests pass (3 new).