refactor(gateway): move egress entrypoint into gateway/egress

egress_entrypoint.sh is the mitmdump launcher for the egress data plane,
so it belongs with the gateway egress service, not at the package root.
Moved to bot_bottle/gateway/egress/entrypoint.sh (prefix stripped now
that the package namespaces it).

Only the Dockerfile.gateway COPY source changes; the runtime target
(/app/egress-entrypoint.sh, referenced by bootstrap's egress DaemonSpec)
is unchanged, and the infra images inherit it via FROM gateway. Updated
the test that resolves the script path and the infra-artifact docstring
example. Full unit suite green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/firecracker/infra_artifact.py

@@ -66,7 +66,7 @@ def infra_artifact_version(init_script: str, *, repo_root: Path = _REPO_ROOT) ->
 
     The package is `COPY bot_bottle /app/bot_bottle`'d wholesale into the image,
     so hash *every* regular file under it — not just `*.py`. Non-Python inputs
-    (e.g. `egress_entrypoint.sh`, `netpool.defaults.env`) are baked in too, and
+    (e.g. `gateway/egress/entrypoint.sh`, `netpool.defaults.env`) are baked in too, and
     a change to one must bump the version or a launch host could boot a stale
     rootfs whose code differs from its checkout. `__pycache__`/`.pyc` are the
     only exclusions — build artifacts, never copied."""