Assistant node, and the Control tab as one conversation (openpanel V5)

run_turn takes its tools, system prompt and tool caller; the chat passes the
MCP tools as before. The assistant node offers one tool per wired target,
reads its other inputs as context, streams its reply a sentence at a time,
warms the provider on `listening` and says so when the provider is not there.
Completions share the per-server HTTP client. wren replies when nothing reads
`unhandled`, stt shows a note when nothing was heard. /ai/control lists one
conversation per entry node with the messages to watch.

backend/fluksio/flow/controller.py

@@ -41,6 +41,7 @@ from fluksio.flow.executor import ExecutionService
 from fluksio.flow.messages import MessageSpec, flow_of, qualify
 from fluksio.flow.nodes import (
     RESERVED_SETTINGS,
+    AssistantNode,
     ChangeNode,
     DebounceNode,
     DelayNode,
@@ -385,6 +386,12 @@ NODE_TYPES: dict[str, NodeType] = {
         cls=WrenNode,
         params_schema=_schema_of(WrenNode),
     ),
+    "assistant": NodeType(
+        title="Assistant",
+        description="Answer a request with the AI provider, acting on what is wired to it.",
+        cls=AssistantNode,
+        params_schema=_schema_of(AssistantNode),
+    ),
     "stt": NodeType(
         title="Speech to text",
         description="Turn a microphone's audio into text, through a speech server.",