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

tests/unit/test_egress_addon_core.py

@@ -951,7 +951,7 @@ class TestScanOutbound(unittest.TestCase):
             body='{"jsonrpc":"2.0","method":"initialize"}',
         )
         self.assertIsNone(scan_outbound(route, text, {
-            "EGRESS_TOKEN_0": "sidecar-owned-secret",
+            "EGRESS_TOKEN_0": "gateway-owned-secret",
         }))
 
     def test_token_in_body_blocked(self):
@@ -1302,7 +1302,7 @@ class TestScanOutboundEnhanced(unittest.TestCase):
         self.assertEqual("warn", result.severity)
 
     def test_bot_bottle_sensitive_prefixes_env_var(self):
-        # When the sidecar env contains BOT_BOTTLE_SENSITIVE_PREFIXES,
+        # When the gateway env contains BOT_BOTTLE_SENSITIVE_PREFIXES,
         # scan_outbound should scan those additional prefixes.
         secret = "extra-sensitive-value-abc"
         env = {
@@ -1324,7 +1324,7 @@ class TestScanOutboundEnhanced(unittest.TestCase):
         self.assertIsNotNone(result)
 
     def test_canary_detected_via_random_secret_env_name(self):
-        # The fake secret uses a randomized env name that the sidecar marks
+        # The fake secret uses a randomized env name that the gateway marks
         # as sensitive through BOT_BOTTLE_SENSITIVE_PREFIXES.
         canary = "canaryvalue12345abcdef"
         env = {