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/constants.py

@@ -0,0 +1,17 @@
+"""Shared wire-protocol constants for gateway-bundled modules.
+
+Single source of truth for values that appear across the egress addon,
+git-http backend, supervise server, and git-gate renderer. Importing
+from this module instead of duplicating the literals means a rename is
+a one-line change and is caught by the type checker at the import site."""
+
+# App-layer identity token header. Delivered as proxy credentials
+# (HTTPS_PROXY=http://<bottle_id>:<token>@gw) by launch; the egress
+# addon reads and strips it, the supervise server and git-http backend
+# read it for attribution, and none of them forward it upstream.
+IDENTITY_HEADER = "x-bot-bottle-identity"
+
+# Shared timeout (seconds) for all git-gate subprocess and CGI calls:
+# git daemon (--timeout/--init-timeout), the access-hook subprocess in
+# git_http_backend, and the git http-backend CGI subprocess.
+GIT_GATE_TIMEOUT_SECS = 15