fix: keep the root barrel edge-safe (1.2.1)

[vreshch]

Why

1.1.0 added serveHealth to the root barrel with a static import ... from 'node:http'. Next bundles instrumentation.ts for the edge runtime whenever a middleware exists, and both admin and dashboard import log from the root barrel there. On the edge runtime node:http does not exist, so the middleware chunk throws at module evaluation and every non-/health request 500s. Friday's dependabot bump (^1.0.0 -> ^1.1.0) took admin.agentage.io and dashboard.agentage.io down.

What

• serve-health.ts: type-only import; createServer comes from process.getBuiltinModule('node:http') inside serveHealth, so the barrel loads anywhere and the builtin is only touched when a worker actually calls it. Node >=22.13 is already the engine floor.
• test/unit/edge-safe-barrel.test.ts: walks the static import graph from src/index.ts and fails on any node: builtin the edge runtime lacks (async_hooks and events allow-listed). Guards against the next barrel addition doing this again.
• Version 1.2.1.

Verified

npm run verify green: 441 tests, dist/bootstrap/stdio smokes. Follow-up PRs bump admin + dashboard to 1.2.1.