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

tests/unit/test_git_gate.py

@@ -218,9 +218,9 @@ class TestHookRender(unittest.TestCase):
         self.assertIn("supervisor approved # gitleaks:allow", hook)
         self.assertIn("supervisor rejected # gitleaks:allow", hook)
 
-    def test_inline_gitleaks_allow_python_imports_work_in_sidecar_layout(self):
+    def test_inline_gitleaks_allow_python_imports_work_in_gateway_layout(self):
         hook = git_gate_render_hook()
-        # The sidecar image copies supervise.py flat under /app, while
+        # The gateway image copies supervise.py flat under /app, while
         # host-side tests import it through the bot_bottle package.
         # Hooks execute from the bare repo directory, so the embedded
         # Python must include /app and support both import layouts.