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.py

@@ -1,6 +1,6 @@
 """Per-agent git-gate (PRD 0008).
 
-A third per-agent sidecar that fronts the bottle's declared git
+A third per-agent daemon that fronts the bottle's declared git
 upstreams as a transparent mirror. Each `bottle.git` entry maps to
 a bare repo on the gate; `git daemon` serves the bare repos over
 `git://<gate>/<name>.git`. Two hooks make the mirror bidirectional:
@@ -15,7 +15,7 @@ a bare repo on the gate; `git daemon` serves the bare repos over
 
 The agent never sees the upstream credential under either path.
 
-Why a separate sidecar (not folded into egress or ssh-gate): the
+Why a separate daemon (not folded into egress or ssh-gate): the
 gate is the only one of the three that holds upstream push
 credentials. Mixing it with egress would put push creds in the
 same blast radius as internet-facing TLS interception; mixing it
@@ -23,7 +23,7 @@ with ssh-gate would force ssh-gate above L4 and into git-protocol
 land. See `docs/prds/0008-git-gate.md`.
 
 This module defines the abstract gate (`GitGate`) and its plan
-dataclass (`GitGatePlan`). The sidecar's start/stop lifecycle is
+dataclass (`GitGatePlan`). The gateway's start/stop lifecycle is
 backend-specific and lives on concrete subclasses (see
 `bot_bottle/backend/docker/git_gate.py`)."""
 
@@ -86,7 +86,7 @@ class GitGatePlan:
 
 class GitGate(ABC):
     """The per-agent git-gate. Encapsulates the host-side prepare
-    (upstream lift + entrypoint/hook render); the sidecar's
+    (upstream lift + entrypoint/hook render); the gateway's
     start/stop lifecycle is backend-specific and lives on concrete
     subclasses."""