feat: add session persistence to CLI

- Add save-only endpoint (POST /conversations/{id}/save) for persisting
  messages without triggering agent execution
- Add sessions command to list previous conversation sessions
- Add --resume flag to chat command for resuming sessions by ID
- Buffer streamed responses and save after completion
- Update AGENTS.md with session commands and remove outdated limitation
- Add 3 new tests for save endpoint (208 total)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/tests/test_conversations.py

@@ -297,3 +297,87 @@ class TestSummarization:
 
         assert "[Previous Summary]" in formatted
         assert "Previous context summary" in formatted
+
+
+class TestSaveMessagesAPI:
+    """Tests for the save messages endpoint (no agent execution)."""
+
+    @pytest.mark.anyio
+    async def test_save_messages(self, auth_client):
+        """Test saving a message pair without triggering agent."""
+        # First create a conversation
+        response = await auth_client.post(
+            "/conversations/",
+            json={"agent_type": "task", "working_dir": "."}
+        )
+        assert response.status_code == 201
+        conv_id = response.json()["id"]
+
+        # Save a message pair
+        response = await auth_client.post(
+            f"/conversations/{conv_id}/save",
+            json={
+                "user_content": "Find all Python files",
+                "assistant_content": "I found 5 Python files in the project.",
+            }
+        )
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["user_message"]["role"] == "user"
+        assert data["user_message"]["content"] == "Find all Python files"
+        assert data["assistant_message"]["role"] == "assistant"
+        assert data["assistant_message"]["content"] == "I found 5 Python files in the project."
+        assert data["total_tokens"] > 0
+
+    @pytest.mark.anyio
+    async def test_save_messages_not_found(self, auth_client):
+        """Test saving messages to non-existent conversation."""
+        fake_id = uuid4()
+        response = await auth_client.post(
+            f"/conversations/{fake_id}/save",
+            json={
+                "user_content": "Test",
+                "assistant_content": "Response",
+            }
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.anyio
+    async def test_save_messages_updates_token_count(self, auth_client):
+        """Test that saving messages updates the conversation token count."""
+        # Create conversation
+        response = await auth_client.post(
+            "/conversations/",
+            json={"agent_type": "explore", "working_dir": "."}
+        )
+        conv_id = response.json()["id"]
+        assert response.json()["total_tokens"] == 0
+
+        # Save first message pair
+        response = await auth_client.post(
+            f"/conversations/{conv_id}/save",
+            json={
+                "user_content": "Hello",
+                "assistant_content": "Hi there!",
+            }
+        )
+        first_tokens = response.json()["total_tokens"]
+        assert first_tokens > 0
+
+        # Save second message pair
+        response = await auth_client.post(
+            f"/conversations/{conv_id}/save",
+            json={
+                "user_content": "How are you?",
+                "assistant_content": "I'm doing well, thank you for asking!",
+            }
+        )
+        second_tokens = response.json()["total_tokens"]
+        assert second_tokens > first_tokens
+
+        # Verify via get endpoint
+        response = await auth_client.get(f"/conversations/{conv_id}")
+        assert response.status_code == 200
+        assert response.json()["total_tokens"] == second_tokens
+        assert len(response.json()["messages"]) == 4