Release version management: single VERSION file as source of truth

- VERSION at repo root (0.1.2); bump it to cut a release
- App: generated AppVersion.kt (config-cache-safe Gradle task with
  VERSION as declared input) shown in Settings; sent to the gateway
  via X-Iris-App-Version header on the SSE open
- Gateway: reports its own version in hello.ack server_caps.app_version
  (read from the repo-root VERSION via the plugin symlink); stores the
  app's version in the device registry caps (merge, not overwrite, so
  an old app reconnecting without the header doesn't wipe it)
- Settings: app + gateway version rows, mismatch hint, and a best-effort
  Gitea latest-release check (ReleaseCheck) with an 'update available' hint
- Release workflow: reads VERSION from the repo (no manual input), with
  a guard against an empty file
- Docs: frames.schema.json + 04-wire-protocol.md updated for app_version

gateway-plugin/adapter.py

@@ -135,6 +135,7 @@ from .setup import (
     validate_config,
 )
 from .tool_frames import ToolProgressHandlers
+from .version import plugin_version
 
 logger = logging.getLogger(__name__)
 
@@ -778,6 +779,9 @@ class IrisAdapter(
                 self._push.server_url if isinstance(self._push, NtfyBackend) else ""
             ),
             "pickers": True,  # picker.choice / picker.select (slash-command menus)
+            # Release version from the repo-root VERSION file (version.py);
+            # the app shows it in Settings and hints on app/gateway mismatch.
+            "app_version": plugin_version(),
         }