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.

bot_bottle/policy_resolver.py

@@ -22,8 +22,7 @@ closed too rather than silently serving stale or empty policy.
 
 The resolved value is the policy blob the orchestrator stores verbatim; the
 consumer parses it (e.g. the egress addon's `load_config`). This module is
-stdlib-only and free of bot-bottle imports so it can be COPYed flat into
-the gateway.
+stdlib-only and free of bot-bottle imports.
 """
 
 from __future__ import annotations