feat: add --json flag to skills check

[elliotllliu]

Summary

Fixes #663 — adds a --json flag to skills check for machine-readable output.

Usage

# Machine-readable output
npx skills check --json
# [{"name":"apollo-mcp-server","source":"apollographql/skills"}, ...]

# Human-readable output (unchanged)
npx skills check

CI Example

updates=$(npx skills check --json | jq 'length')
if [ "$updates" -gt 0 ]; then
  npx skills update
fi

Changes

• runCheck() parses --json flag from args
• When set: suppresses ANSI output, prints JSON array of {name, source} objects
• Empty array [] when no skills or no updates
• Help text and examples updated
• Human-readable output completely unchanged without the flag