Let build-apk.sh build either variant: ./build-apk.sh [release|debug]

Dev Updater is gaining build modes declared per component as complete
command lines, so the variant is a positional word and nothing else.
Release stays the default and the only one that signs.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

AGENTS.md

@@ -204,7 +204,9 @@ first if a remote spawn ever mangles an argument.
   to produce the APK to install on a phone (through Dev Updater), or
   `./run-android.sh` to build, install, and launch on the emulator.
   **The phone gets the release build**, signed with a key the script
-  generates once under `~/.config/ai-app/release.jks` (never in the repo).
+  generates once under `~/.config/ai-app/release.jks` (never in the repo);
+  `./build-apk.sh debug` builds the other variant, and Dev Updater's build
+  modes call the script with exactly that word.
   The emulator scripts stay on the debug build; a debuggable build runs
   Compose at a fraction of release speed, so never read a frame time from
   one as the app's -- the render report now says which build it came from.