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/backend/docker/setup.py

@@ -1,7 +1,7 @@
 """Host setup + status for the Docker backend.
 
 Unlike Firecracker, the Docker backend needs no privileged one-time
-host provisioning (no TAP pool / nft table) — networks and the sidecar
+host provisioning (no TAP pool / nft table) — networks and the gateway
 bundle are created per-launch. So `setup()` is mostly an install/daemon
 pointer, and `status()` reports whether docker is usable.
 
@@ -45,7 +45,7 @@ def setup() -> int:
         return 1
     sys.stderr.write(
         "Docker backend: no privileged host setup required — networks and "
-        "the sidecar bundle are created per-launch.\n"
+        "the gateway are created per-launch.\n"
     )
     if not _daemon_reachable():
         sys.stderr.write(
@@ -64,7 +64,7 @@ def setup() -> int:
 def teardown() -> int:
     sys.stderr.write(
         "Docker backend: nothing to undo — it provisions no privileged host "
-        "state (networks and the sidecar bundle are per-launch and are "
+        "state (networks and the gateway are per-launch and are "
         "removed by `./cli.py cleanup`). Docker itself is left installed.\n"
     )
     return 0