feat: replace direct-Postgres checkpointer with HTTP proxy

Replace AsyncPostgresSaver with HttpCheckpointSaver that proxies all
LangGraph checkpoint operations through the agentex backend API.
Agents no longer need DATABASE_URL or a direct Postgres connection.

- Add _http_checkpointer.py: BaseCheckpointSaver subclass using httpx
- Update checkpointer.py: create_checkpointer() returns HttpCheckpointSaver
- Remove langgraph-checkpoint-postgres and psycopg[binary] deps
- Add langgraph-checkpoint (base types only, no DB driver)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: danielmillerp

pyproject.toml

@@ -48,6 +48,7 @@ dependencies = [
     "yaspin>=3.1.0",
     "claude-agent-sdk>=0.1.0",
     "anthropic>=0.40.0",
+    "langgraph-checkpoint>=2.0.0",
 ]
 
 requires-python = ">= 3.12,<4"

src/agentex/lib/adk/__init__.py

@@ -5,6 +5,7 @@
 from agentex.lib.adk._modules.acp import ACPModule
 from agentex.lib.adk._modules.agents import AgentsModule
 from agentex.lib.adk._modules.agent_task_tracker import AgentTaskTrackerModule
+from agentex.lib.adk._modules.checkpointer import create_checkpointer
 from agentex.lib.adk._modules.events import EventsModule
 from agentex.lib.adk._modules.messages import MessagesModule
 from agentex.lib.adk._modules.state import StateModule
@@ -27,16 +28,19 @@
 
 __all__ = [
     # Core
-    "acp", 
+    "acp",
     "agents",
-    "tasks", 
-    "messages", 
-    "state", 
-    "streaming", 
-    "tracing", 
+    "tasks",
+    "messages",
+    "state",
+    "streaming",
+    "tracing",
     "events",
     "agent_task_tracker",
 
+    # Checkpointing
+    "create_checkpointer",
+
     # Providers
     "providers",
     # Utils