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

nix/firecracker-netpool.nix

@@ -2,7 +2,7 @@
 #
 # The one-time privileged setup the Firecracker backend needs: a pool of
 # user- (or group-) owned point-to-point TAP devices plus a fail-closed
-# nftables table that confines every microVM to its own sidecar.
+# nftables table that confines every microVM to its own gateway.
 #
 # NON-INVASIVE BY DESIGN. It does NOT flip `networking.nftables.enable`
 # (which would switch your whole host firewall backend) or
@@ -150,7 +150,7 @@ in
       }
     ];
 
-    # VM->sidecar traffic is DNAT'd and forwarded, so forwarding must be on.
+    # VM->gateway traffic is DNAT'd and forwarded, so forwarding must be on.
     boot.kernel.sysctl."net.ipv4.ip_forward" = 1;
 
     # One oneshot brings up the whole pool (TAPs + independent nft table)