feat(integrations): add Google ADK harness plugin

cascadeflow/__init__.py

@@ -239,6 +239,22 @@
     get_tool_risk_routing,
 )
 
+# NEW: Harness API scaffold (V2 core branch)
+# NOTE: harness.agent is NOT re-exported here — it would shadow the
+# cascadeflow.agent *module* and break dotted-path resolution
+# (e.g. patch("cascadeflow.agent.PROVIDER_REGISTRY")).
+# Use ``from cascadeflow.harness import agent`` instead.
+from .harness import (
+    HarnessConfig,
+    HarnessInitReport,
+    HarnessRunContext,
+    init,
+    reset,
+    run,
+    get_harness_config,
+    get_current_run,
+)
+
 # ==================== MAIN AGENT & RESULT ====================
 
 
@@ -381,6 +397,15 @@
     "ToolRiskClassification",  # NEW: v0.8.0 - Classification result
     "ToolRiskClassifier",  # NEW: v0.8.0 - Tool risk classifier
     "get_tool_risk_routing",  # NEW: v0.8.0 - Routing by risk level
+    # ===== HARNESS API (V2 scaffold) =====
+    "HarnessConfig",
+    "HarnessInitReport",
+    "HarnessRunContext",
+    "init",
+    "reset",
+    "run",
+    "get_harness_config",
+    "get_current_run",
     # ===== PROVIDERS =====
     "ModelResponse",
     "BaseProvider",

cascadeflow/harness/__init__.py

@@ -0,0 +1,34 @@
+"""
+Core harness API scaffold for V2 planning work.
+
+This module provides a minimal, backward-compatible surface:
+- init(): global harness settings (opt-in)
+- run(): scoped run context for budget/trace accounting
+- agent(): decorator for attaching policy metadata
+
+The implementation intentionally avoids modifying existing CascadeAgent behavior.
+"""
+
+from .api import (
+    HarnessConfig,
+    HarnessInitReport,
+    HarnessRunContext,
+    agent,
+    get_current_run,
+    get_harness_config,
+    init,
+    reset,
+    run,
+)
+
+__all__ = [
+    "HarnessConfig",
+    "HarnessInitReport",
+    "HarnessRunContext",
+    "init",
+    "run",
+    "agent",
+    "get_current_run",
+    "get_harness_config",
+    "reset",
+]