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/firecracker/launch.py

@@ -38,10 +38,7 @@ from ...bottle_state import (
     git_gate_state_dir,
     read_committed_image,
 )
-from ...egress import (
-    egress_agent_env_entries,
-    egress_resolve_token_values,
-)
+from ...egress import Egress
 from ...git_gate import GitGate
 from ...image_cache import check_stale_path
 from ...log import die, info, warn
@@ -110,7 +107,7 @@ def launch(
         # (in memory) for the gateway to inject — the agent never sees them.
         # Attribution is by the VM's guest IP (unspoofable via /31 TAP + nft).
         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()
@@ -284,7 +281,7 @@ def _agent_guest_env(plan: FirecrackerBottlePlan, host_ip: str) -> dict[str, str
         env["MCP_SUPERVISE_URL"] = plan.agent_supervise_url
     if plan.env_var_secret:
         env[ENV_VAR_SECRET_NAME] = plan.env_var_secret
-    for entry in egress_agent_env_entries(plan.egress_plan):
+    for entry in Egress().agent_env_entries(plan.egress_plan):
         key, _, value = entry.partition("=")
         env[key] = value
     env.update(plan.agent_provision.guest_env)