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

docs/demo.tape

@@ -38,7 +38,7 @@ Type "y"
 Enter
 
 # Wait for the bottle to launch: networks created, pipelock + git-gate
-# sidecars started, agent container started, claude boots.
+# companion containers started, agent container started, claude boots.
 Sleep 22s
 
 # Probe 1 — warm-up. A reply at all proves api.anthropic.com is
@@ -72,7 +72,7 @@ Type "init /tmp/r, commit AKIAQRJHK7N5ZPM2VXTL to leak.txt, push to ssh://git@up
 Enter
 Sleep 30s
 
-# Leave claude. The launcher tears down the container, sidecars, and
+# Leave claude. The launcher tears down the container, companion containers, and
 # networks on session end.
 Ctrl+D
 Sleep 4s