refactor(gateway): replace flat-file import shims with installed package

Install bot_bottle via pip in Dockerfile.gateway instead of COPYing
individual .py files flat under /app/. This eliminates the try/except
import shims in egress_addon_core, dlp_detectors, egress_addon,
supervise, supervise_server, and git_http_backend that existed only
to support the flat-bundle layout.

Adds bot_bottle/constants.py as a single source of truth for
IDENTITY_HEADER and GIT_GATE_TIMEOUT_SECS, removing the duplicated
literal definitions in egress_addon.py, supervise_server.py,
git_http_backend.py, and git_gate_render.py.

Test files updated to match: test_supervise_server.py drops the
sys.path.insert hack in favour of direct package imports; the
egress_addon test shims no longer pre-populate sys.modules with a
bare egress_addon_core alias.

pyproject.toml

@@ -0,0 +1,8 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.backends.legacy:build"
+
+[project]
+name = "bot-bottle"
+version = "0.0.0"
+requires-python = ">=3.11"