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/ARCHITECTURE.md

@@ -41,6 +41,9 @@ Your app derives from it:
   `setDisplayName()`, `setQuickStyle()`, `setQmlModule()`). The app's own command-line
   options go to the base constructor as a list of `QCommandLineOption`s; they are parsed and
   listed in `--help` with the built-in ones, and read with `isOptionSet()`/`optionValue()`.
+  Its commands (flags such as `--ha-remove`) go there as a second list: like the manifest
+  commands they get a `QCoreApplication` (no display needed, works over SSH), and `exec()`
+  runs `prepare()` and then `runCommand(name)` in mode `Command` instead of a mode.
 - **Hooks:**
   - `prepare(QString* error)`: first, once mode and app key are known, before the QML
     engine, the surfaces and SteamVR exist (not for manifest commands). Returning `false`
@@ -48,10 +51,13 @@ Your app derives from it:
   - `loading()`: the shared QML engine, the surfaces and their render backends exist; QML
     isn't loaded yet. Set up what the QML needs here.
   - `started()`: everything runs. Connect to surfaces and input here.
+  - `runCommand(name)`: one of the app's commands, after `prepare()`; no QML engine,
+    surfaces or SteamVR. Its result is the exit code.
 - **Accessors:** `mode()`, `appKey()`, `appName()`, `isOptionSet()`, `optionValue()`,
   `engine()`, `surfaces()`, `surface()` (the first), `surface(id)`, `vrRuntime()`,
   `navigation()`.
-- **Lifetime:** the base constructor creates the `QGuiApplication`, so your members can be
+- **Lifetime:** the base constructor creates the `QGuiApplication` (`QCoreApplication` for
+  commands), so your members can be
   QObjects with timers. All QML and surfaces are torn down at the end of `exec()`, while your
   object and its members still exist.