refactor(gateway): split data-plane files into egress/supervisor/git_gate services

Group the gateway's data-plane modules into three service sub-packages
mirroring the host-side trio (bot_bottle.egress / .supervisor / .git_gate):

  gateway/egress/     addon_core, addon, dlp_config, dlp_detectors
  gateway/supervisor/ server            (was supervise_server)
  gateway/git_gate/   render, http_backend

Prefix-stripped filenames now that the package namespaces them; each
sub-package has a thin docstring __init__ (no eager imports, cheap leaf
loads). The two cross-cutting files stay at the gateway root:
policy_resolver (shared per-client lookup) and gateway_init, renamed to
bootstrap now that gateway/ already namespaces it.

Updated all importers (bot_bottle + tests), the in-VM/container `-m`
launch strings, the Dockerfile.gateway addon shim + ENTRYPOINT, and the
five gateway entries in scripts/critical-modules.txt. Full unit suite
green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/integration/test_gateway_image.py

@@ -91,7 +91,7 @@ class TestGatewayImage(unittest.TestCase):
         # Probe that the package imports resolve inside the image.
         rc, out = self._run_in_image(
             "python3", "-c",
-            "from bot_bottle.supervisor import types; from bot_bottle.gateway import supervise_server; print('ok')",
+            "from bot_bottle.supervisor import types; from bot_bottle.gateway.supervisor import server as supervise_server; print('ok')",
         )
         self.assertEqual(0, rc, msg=out)
         self.assertIn("ok", out)