docs(examples): one-command docker-compose quickstart

[prashar32]

Lowers time-to-first-value: examples/quickstart-compose is a docker compose up demo that shows RiskKernel hard-stopping a runaway agent — no API key, no local Go/Python.

What it does

cd examples/quickstart-compose && docker compose up

  call 1  -> 200 OK    tokens=1600  cost=$0.007000
  ...
  call 5  -> 200 OK    tokens=1600  cost=$0.007000
  call 6  -> HTTP 402   RiskKernel HALTED the run:
      {"code":"loop_budget_exceeded","message":"run halted: loop_budget_exceeded"}

How (three tiny services)

• mock-llm (nginx:alpine) — a stand-in OpenAI-compatible upstream returning one canned completion with token usage, so the demo needs no real provider or key.
• riskkernel — the daemon with a hard RISKKERNEL_DEFAULT_LOOPS=5 (other dimensions unlimited, so the halt reason is unambiguous), forwarding to the mock via RISKKERNEL_OPENAI_BASE_URL.
• sample-agent (curlimages/curl) — a minimal shell loop that calls the proxy under one run id and never stops on its own; RiskKernel stops it.

A short README shows how to swap the mock for a real provider (still one env var on the app side).

Notes

• The daemon service sets pull_policy: always — during testing a stale cached :latest (a pre-release v0.2.0 dev build) made the override silently no-op, so the demo must guarantee a fresh image.
• gpt-4o is in the pricing table, so the demo also shows real per-call cost as a bonus; the loop budget is the deterministic killer either way.

Verified

Brought the stack up end-to-end (docker compose up --exit-code-from sample-agent): calls 1–5 metered at 1600 tokens each, call 6 halted with loop_budget_exceeded, compose exit code 0, no API key. docker compose config validates and sh -n agent.sh is clean.

Closes #95