feat: add npm update notifier

[jlitola]

Summary

• add a background npm dist-tags update check for sticky CLI installs
• cache non-secret update-check state under XDG config and print stale notices to stderr only
• skip CI, non-TTY stderr, help/version, ephemeral package runners, and MCP stdio invocations
• keep package/plugin version metadata at the currently released 0.2.3 value

Notes

• stale-version notices intentionally print on every eligible invocation once a newer npm latest is known
• update-check cache now uses ~/.config/githits; auth/client storage still uses ~/.githits and should be migrated in a follow-up before public release
• lint currently exits 0 but reports existing ToolDefinition<any, any> warnings in src/commands/mcp.ts; follow-up will clean those up

Verification

• bun test
• bun run typecheck
• bun run build
• bun run lint