refactor(egress): make Egress a service class in an egress package

Split the flat egress.py into an egress/ package: neutral DTOs
(EgressRoute, EgressPlan) in plan.py, and the concrete Egress service
class plus rendering/env helpers in service.py, behind a thin
__getattr__ facade so leaf imports stay cheap.

Egress drops ABC and becomes a concrete service the backends call:
resolve_token_values / agent_env_entries / prepare are now methods.
Backend launch paths (docker, firecracker, macos_container) call
Egress().<method>(...) instead of the module-level functions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/docker/launch.py

@@ -37,7 +37,7 @@ from pathlib import Path
 from typing import Callable, Generator
 
 from ...agent_provider import runtime_for
-from ...egress import egress_resolve_token_values
+from ...egress import Egress
 from ...git_gate import GitGate
 from ...image_cache import check_stale
 from ...log import die, info, warn
@@ -143,7 +143,7 @@ def launch(
         # handed to the orchestrator (in memory) for the gateway to inject —
         # the agent never sees them.
         effective_env = {**os.environ, **plan.agent_provision.provisioned_env}
-        token_values = egress_resolve_token_values(
+        token_values = Egress().resolve_token_values(
             plan.egress_plan.token_env_map, effective_env,
         )
         teardown_timeout = resolve_teardown_timeout()