codeSplitters dependency doctor: run tests on Cloudflare in the sandbox container #197

Follow-up to #194. Today tests only run on the desktop (localTester in src/deps.ts). On the edge the doctor still opens a branch, but its updates are marked untested.

Add a Tester backed by the AGENT_SANDBOX container: clone, bun install, test script, tear down. Keep it admin-only and off by default, like hosted agents, because it runs repo code and costs container time.

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