feat(cli): add version check system with agent-friendly output

- New `updateCheck.ts` utility: cached npm registry check (24h TTL),
  sync `getUpdateMeta()` for _meta envelope, `printUpdateNotice()` for
  passive stderr banner
- `upgrade --check --json`: machine-readable version check for AI agents
  Returns { current, latest, updateAvailable }
- `_meta` envelope on all --json commands (info, lint, benchmark,
  compositions): includes version, latestVersion, updateAvailable
- `doctor` shows version check as first row
- Passive update notice on stderr after command completes (skipped in
  CI, non-TTY, --json, --quiet)
- Background check fires on startup (non-blocking, populates cache)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/commands/lint.ts

@@ -3,6 +3,7 @@ import { readFileSync } from "node:fs";
 import { lintHyperframeHtml } from "@hyperframes/core/lint";
 import { c } from "../ui/colors.js";
 import { resolveProject } from "../utils/project.js";
+import { withMeta } from "../utils/updateCheck.js";
 
 export default defineCommand({
   meta: { name: "lint", description: "Validate a composition for common mistakes" },
@@ -16,7 +17,7 @@ export default defineCommand({
     const result = lintHyperframeHtml(html, { filePath: project.indexPath });
 
     if (args.json) {
-      console.log(JSON.stringify(result, null, 2));
+      console.log(JSON.stringify(withMeta(result), null, 2));
       process.exit(result.ok ? 0 : 1);
     }