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/unit/test_gateway_init.py

@@ -1,6 +1,6 @@
 """Unit: gateway data-plane init supervisor (PRD 0070; PRD 0024 bundle shape).
 
-Tests both the helper functions in `bot_bottle.gateway.gateway_init`
+Tests both the helper functions in `bot_bottle.gateway.bootstrap`
 and the supervisor's end-to-end signal / exit-code behavior. The
 end-to-end tests use real subprocesses (`sleep`, `/bin/sh -c '...'`) —
 short-lived, no docker required — so they run under `tests/unit/`
@@ -18,7 +18,7 @@ import warnings
 from pathlib import Path
 from unittest.mock import patch
 
-from bot_bottle.gateway.gateway_init import (
+from bot_bottle.gateway.bootstrap import (
     _DaemonSpec,
     _Supervisor,
     _argv_for_daemon,
@@ -489,7 +489,7 @@ class TestSupervisor(unittest.TestCase):
         time.sleep(0.3)  # let `trap` register
         sup.request_shutdown(reason="test")
 
-        with patch("bot_bottle.gateway.gateway_init._GRACE_SECONDS", 0.3):
+        with patch("bot_bottle.gateway.bootstrap._GRACE_SECONDS", 0.3):
             rc = self._drive(sup, max_wait_s=4.0)
 
         # Process was SIGKILL'd → returncode -9 on POSIX.
@@ -531,7 +531,7 @@ class TestMainEndToEnd(unittest.TestCase):
 
         helper = (
             "import os, runpy, sys\n"
-            "from bot_bottle.gateway import gateway_init as si\n"
+            "from bot_bottle.gateway import bootstrap as si\n"
             "si._DAEMONS = (\n"
             f"  si._DaemonSpec('alpha', ({SLEEP!r},'30')),\n"
             f"  si._DaemonSpec('beta',  ({SLEEP!r},'30')),\n"