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/backend/firecracker/netpool.py

@@ -18,7 +18,7 @@ Topology (per slot i):
   * a /31 host<->guest link: host = base + 2i (the gateway the VM
     routes through), guest = base + 2i + 1 (the VM's address).
   * isolation via ``table inet bot_bottle_fc``: a VM reaches only its
-    own sidecar (DNAT'd from the host TAP IP) and nothing else.
+    own gateway (DNAT'd from the host TAP IP) and nothing else.
 
 The default IP block is ``10.243.0.0/16`` — an intentionally obscure
 corner of RFC-1918 private space. RFC-1918 is the range *designated*
@@ -88,7 +88,7 @@ def ip_base() -> str:
     return _cfg("BOT_BOTTLE_FC_IP_BASE")
 
 
-# Sidecar ports the VM reaches at its host-side TAP IP. Kept in sync
+# Gateway ports the VM reaches at its host-side TAP IP. Kept in sync
 # with the backend constants (egress 9099, supervise 9100, git-http
 # 9420); rendered into the setup output for operator visibility.
 GATEWAY_PORTS = (9099, 9100, 9420)