feat: update check notifications — non-blocking version detection

[flyingrobots]

Summary

Add non-blocking update check that notifies users when a newer version is available on npm.

• src/update-check.js — check npm registry, cache result, format notification
• Fire-and-forget pattern: check runs during command execution, notification shows from previous cache
• 24h cache TTL in ~/.gitmind/update-check.json
• 3s fetch timeout via AbortController
• All errors silently swallowed — must never break any command
• Notification on stderr, suppressed in --json mode
• No new runtime dependencies (uses Node built-in fetch)

Acceptance Criteria

• Update check never blocks or slows commands
• Notification appears on stderr only when a newer version exists
• --json output is never polluted
• Tests cover cache read/write, semver comparison, error handling