App commands and --ha-remove (iteration 2, step 6)

The platform accepts app commands (D43): flags that get a
QCoreApplication like the manifest commands and run prepare() and then
the new hook runCommand(). The app's --ha-remove clears the device's
retained topics, so HA deletes the device; uninstall.sh runs it after
stopping the app and before unregistering from SteamVR.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PROGRESS.md

@@ -25,7 +25,8 @@ CLAUDE.md ("Working rules").
 - **Implementation** follows the order in DESIGN.md (steps 1–7). Steps 1 (libmosquitto in
   CMake and the containers), 2 (`MqttClient`, `MosquittoClient`, `tst_mqtt`) 3
   (`HaEntity`, `HaSensor`, `HaDevice`, unit tests) and 4 (app wiring, `--desktop-novr`,
-  UI-1) and 5 (TLS) done; next: step 6 (app commands, `--ha-remove`, `uninstall.sh`).
+  UI-1) 5 (TLS) and 6 (app commands, `--ha-remove`, `uninstall.sh`) done; next: step 7
+  (packaging check, docs).
 
 ## Next steps
 
@@ -178,6 +179,12 @@ Known limitation: the bundled-Qt package does not support WebEngine in this iter
     validation messages.
   - Still open: which CA store the Frame's OpenSSL uses by default (for `ca_file` empty);
     check on the device.
+- Step 6: app commands in the platform (D43: second option list, `QCoreApplication`, mode
+  `Command`, hook `runCommand()`; `tst_surfaces` `surfaces_command`); `--ha-remove` in the
+  app; `uninstall.sh` stops the app, runs `--ha-remove` (failure doesn't stop it) and
+  unregisters. Checked end to end against a local mosquitto, without a display: retained
+  discovery, availability, state and entity availability before, none after (exit 0);
+  broker down → exit 1 after 10 s; missing settings → exit 1.
 
 ### 2026-10-05: requirements approved