Workers screen shows each agent's version and protocol, and who is behind

The hello carries the fluksio-worker version. An agent refused for an older
protocol stays listed as Needs upgrade until it attaches again. Docs gain an
Upgrading workers section.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

worker/fluksio_worker/agent.py

@@ -28,6 +28,7 @@ import argparse
 import asyncio
 import contextlib
 import glob
+import importlib.metadata
 import ipaddress
 import json
 import logging
@@ -167,6 +168,7 @@ class Agent:
                     {
                         "op": "hello",
                         "protocol": PROTOCOL,
+                        "version": _version(),
                         "name": self.args.name,
                         "labels": self.args.labels,
                         "python": self.args.python,
@@ -421,6 +423,14 @@ def _remap_cuda(mask: str, requested: str) -> str:
     return ",".join(out)
 
 
+def _version() -> str:
+    """The installed fluksio-worker, or empty when this file was copied over."""
+    try:
+        return importlib.metadata.version("fluksio-worker")
+    except importlib.metadata.PackageNotFoundError:
+        return ""
+
+
 def _venv_digest(python: str) -> str:
     """What is installed here, so the engine can say when it has drifted."""
     try: