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_entrypoint.sh

@@ -1,8 +1,8 @@
 #!/bin/sh
-# Egress daemon entrypoint inside the sidecar bundle (PRD 0024).
+# Egress daemon entrypoint inside the gateway (PRD 0024).
 #
 # Extracted verbatim from Dockerfile.egress's prior inline `sh -c`
-# ENTRYPOINT so the supervisor in bot_bottle/sidecar_init.py can
+# ENTRYPOINT so the supervisor in bot_bottle/gateway_init.py can
 # call it as a normal child. Behavior is unchanged:
 #
 #   * Upstream proxy: when EGRESS_UPSTREAM_PROXY is set, switch
@@ -22,7 +22,7 @@ set -e
 
 # Pin mitmproxy's config dir to the bind-mount location of its CA
 # regardless of which user mitmdump runs as. In the legacy
-# four-sidecar setup (Dockerfile.egress, USER mitmproxy) this
+# four-daemon setup (Dockerfile.egress, USER mitmproxy) this
 # resolved naturally to `~mitmproxy/.mitmproxy`. In the PRD 0024
 # bundle (USER root) `~root/.mitmproxy` is empty, so without this
 # flag mitmdump would generate a fresh CA on the wrong path and