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/git_gate_render.py

@@ -14,18 +14,12 @@ import shlex
 from dataclasses import dataclass
 from pathlib import Path
 
+from .constants import GIT_GATE_TIMEOUT_SECS, IDENTITY_HEADER
 from .manifest import ManifestBottle, ManifestGitEntry
 
 # Short network alias for git-gate inside the gateway. The
 # agent's `.gitconfig` insteadOf rewrites resolve through this name.
 GIT_GATE_HOSTNAME = "git-gate"
-# App-layer identity token header the agent's git sends to git-http and the
-# gateway validates (mirrors egress_addon / git_http_backend IDENTITY_HEADER).
-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
 
 
 @dataclass(frozen=True)