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/egress.py

@@ -2,7 +2,7 @@
 
 This module defines the abstract proxy (`Egress`), its plan
 dataclass (`EgressPlan`), and the resolved per-route shape
-(`EgressRoute`). The sidecar's start/stop lifecycle is backend-
+(`EgressRoute`). The gateway's start/stop lifecycle is backend-
 specific and lives on concrete subclasses (see
 `bot_bottle/backend/docker/egress.py`).
 """
@@ -87,7 +87,7 @@ class EgressRoute(Route):
 
     Inherits `host`, `matches`, `auth_scheme`, and `token_env`
     from `egress_addon_core.Route` — those are the fields that cross the
-    YAML wire into the sidecar. The fields below are host-only and
+    YAML wire into the gateway. The fields below are host-only and
     are never serialised to the addon.
 
     `token_ref` is the host env var the CLI reads at launch and forwards
@@ -386,7 +386,7 @@ class Egress(ABC):
         routes_path.write_text(egress_render_routes(routes, log=log))
         routes_path.chmod(0o600)
         # Generate a per-session fake secret under a plausible random env name.
-        # The sidecar marks that exact env name as sensitive for known-secret
+        # The gateway marks that exact env name as sensitive for known-secret
         # scanning; the agent receives the same name/value as exfil bait.
         canary = secrets.token_urlsafe(32)
         return EgressPlan(