refactor(de-sidecar): purge the "sidecar" name from live code, tests, and current docs

Completes the de-sidecar cleanup: no live code, test, current doc,
script, or nix file mentions or is named "sidecar" any more. Only the
dated PRD/research docs keep the term as historical record (agreed on
the #385 thread).

- Rename `sidecar_init.py`→`gateway_init.py` was done earlier; this pass
  sweeps the remaining descriptive uses: the egress / git-gate / supervise
  components are the gateway's *daemons*, the shared container is the
  *gateway*, the old per-bottle container was the *companion container*.
- Rename `tests/integration/test_sidecar_bundle_image.py`→`test_gateway_image.py`
  and its class; update `docs/ci.md` + `tests/README.md` for the renamed/
  removed integration tests.
- `SIDECAR_PORTS` shell var in `scripts/firecracker-netpool.sh`→`GATEWAY_PORTS`.

Full unit suite green (bar the pre-existing `/bin/sleep`-missing env
errors in test_gateway_init); docker integration — gateway singleton,
broker, real two-bottle multitenant isolation, and the gateway-image
build — all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/orchestrator/docker_broker.py

@@ -2,12 +2,12 @@
 
 On a verified launch request it starts a Docker container; on teardown it
 removes it. This proves the orchestrator -> backend seam on the cheapest
-backend (the sidecar bundle is already containers). Only the request's
+backend (the gateway is already containers). Only the request's
 static ids/flags reach `docker`, so nothing free-form crosses the boundary.
 
 Slice 3 launches a single container from the request's `image_ref`, named
 after the bottle id and labelled for cleanup. Wiring the full agent +
-sidecar bundle (networks, mounts, the consolidated sidecar) is a later
+gateway (networks, mounts, the consolidated gateway) is a later
 slice — this is the seam, not the finished launcher.
 """