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/git_gate_render.py

@@ -1,7 +1,7 @@
 """Pure host-side rendering for the per-agent git-gate (PRD 0008).
 
 Builds the agent's `.gitconfig` insteadOf rewrites, the known_hosts
-line, and the entrypoint / pre-receive / access-hook scripts the sidecar
+line, and the entrypoint / pre-receive / access-hook scripts the gateway
 runs. No docker or forge calls — exposed for tests and reuse across
 backends. Split out of `git_gate.py` so the control surface (`GitGate`)
 and the deploy-key lifecycle (`git_gate_provision`) each read on their
@@ -16,7 +16,7 @@ from pathlib import Path
 
 from .manifest import ManifestBottle, ManifestGitEntry
 
-# Short network alias for git-gate inside the sidecar bundle. The
+# Short network alias for git-gate inside the gateway. The
 # agent's `.gitconfig` insteadOf rewrites resolve through this name.
 GIT_GATE_HOSTNAME = "git-gate"
 # Shared timeout (seconds) for all git-gate subprocess and CGI calls:
@@ -38,7 +38,7 @@ class GitGateUpstream:
     KnownHostKey string from the manifest; the gate's start step
     materialises it into a known_hosts file if non-empty.
 
-    the gate credential paths inside the running sidecar."""
+    the gate credential paths inside the running gateway."""
 
     name: str
     upstream_url: str