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/manifest_agent.py

@@ -17,7 +17,7 @@ class ManifestAgentProvider:
 
     `template` selects a built-in launch/runtime contract. `dockerfile`
     optionally points at a custom agent-image Dockerfile while leaving
-    bot-bottle's sidecar infrastructure intact.
+    bot-bottle's gateway infrastructure intact.
 
     `auth_token` names the host env var that holds the provider's OAuth
     token (Claude only). The provisioner injects a provider-owned egress
@@ -26,7 +26,7 @@ class ManifestAgentProvider:
     so the Claude Code CLI starts.
 
     `forward_host_credentials` forwards the host Codex auth token into
-    the egress sidecar (Codex only).
+    the egress daemon (Codex only).
     """
 
     template: str = "claude"