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/paths.py

@@ -10,7 +10,7 @@ suite points it at a throwaway dir instead of monkey-patching the function
 override covers them all), and operators can relocate the root if needed.
 
 This module has no bot-bottle imports, so it is safe to import from any
-layer (and to COPY flat into the sidecar bundle).
+layer (and to COPY flat into the gateway).
 """
 
 from __future__ import annotations
@@ -35,7 +35,7 @@ def host_db_path() -> Path:
 
     Kept in its own `db/` subdirectory (not directly under the root) so a
     backend that can only bind-mount *directories* can share this one file
-    with a sidecar without exposing the root's other contents (git-gate
+    with a gateway without exposing the root's other contents (git-gate
     keys, per-bottle state, ...)."""
     return bot_bottle_root() / "db" / HOST_DB_FILENAME