make smoke-docker builds the image, stands up a stub upstream + the container
on a shared network, and asserts forward+prefix-strip, upstream-header cleared,
421 on missing header, and 403 + reject sentinel on allowlist miss. Self-
contained (no tailnet), idempotent, local-only (Docker) like cargo-check.
Retire HONCHO_UPSTREAM and the same-origin sentinel; document the per-request
X-Honcho-Upstream header model, OPENCONCHO_DEFAULT_HONCHO_URL seeding, and the
optional OPENCONCHO_UPSTREAM_ALLOWLIST SSRF guard across compose, README,
AGENTS.md, and docs/docker.md.
