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

AGENTS.md

@@ -8,10 +8,10 @@ broad permissions inside a sandbox, so a misbehaving agent cannot reach the
 host. A Python CLI (entry point `cli.py`, package `bot_bottle/`) orchestrates
 the runtime lifecycle and the copying of skills and env vars into it.
 The default backend on compatible macOS hosts is macos-container:
-agents and sidecar bundles run through Apple's `container` CLI without
+agents and gateways run through Apple's `container` CLI without
 requiring Docker. On KVM-capable Linux hosts the default is firecracker:
 agents run in a Firecracker microVM reached over SSH on a point-to-point
-TAP, while the sidecar bundle still uses Docker. The legacy Docker
+TAP, while the gateway still uses Docker. The legacy Docker
 backend remains available with `BOT_BOTTLE_BACKEND=docker` or
 `--backend=docker`.
 
@@ -59,7 +59,7 @@ backend remains available with `BOT_BOTTLE_BACKEND=docker` or
   in a PRD, research note, or decision record.
 - Low dependencies by default. The project is Python, stdlib-first (no
   runtime pip dependencies in the package itself; the only language
-  runtime is the Python 3.13 used by the CLI + sidecars). Ask before
+  runtime is the Python 3.13 used by the CLI + companion containers). Ask before
   adding new tools, runtimes, or package managers.
 - Commit messages follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/):
   `<type>[(scope)][!]: <description>`, where `<type>` is one of `feat`, `fix`,