Add Prefect worker example

[opencolin]

Adds examples/prefect-tenki: the prefect-tenki worker runs each Prefect 3.x flow run in its own microVM (prefect work-pool create --type tenki). Not on PyPI yet, so requirements pin a git SHA.

A real flow run isn't CI-feasible — the guest has to reach a Prefect API, and a localhost server is unreachable from the microVM — so verify.py mirrors upstream's own smoke test: TenkiWorker.run() end-to-end against live Tenki with no server, asserting exit 0 and teardown. Passed live. The README is explicit about what a real deployment needs (a Prefect-equipped image with outbound access).

Root README: Prefect row in Developer tools.

🤖 Generated with Claude Code

[tenki-reviewer]

Review complete. No issues found — approved ✅.

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This PR introduces a new example directory that demonstrates running Prefect workers inside Tenki microVMs. It is additive example content with no production-code changes. The Python script parses config lines, resolves the workspace, and exits non-zero on any failure; the Node shim proxies child exit status. No correctness, concurrency, security, or performance defects were found that meet the review confidence bar.

Files	Change
examples/prefect-tenki/requirements.txt	Pins tenki and prefect (git commit) for the example environment.
examples/prefect-tenki/verify.py	Parses worker config, resolves workspace, runs verification, exits 1 on failure.
examples/prefect-tenki/verify.mjs	Spawns the Python verify script and propagates its exit status.
README.md, examples/prefect-tenki/README.md	Document the example's setup, verification flow, and usage.

_{Reviewed commit: 552ea0c}