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>

tests/unit/test_egress.py

@@ -10,7 +10,6 @@ from bot_bottle.egress import (
     Egress,
     EgressPlan,
     EgressRoute,
-    _yaml_str_escape,
     egress_agent_env_entries,
     egress_manifest_routes,
     egress_render_routes,
@@ -19,6 +18,7 @@ from bot_bottle.egress import (
     egress_gateway_env_entries,
     egress_token_env_map,
 )
+from bot_bottle.egress.service import _yaml_str_escape
 from bot_bottle.errors import MissingEnvVarError
 from bot_bottle.manifest import ManifestIndex
 from bot_bottle.yaml_subset import parse_yaml_subset