build: guard local gradle runs against exhausting the box

The devshell `gradle` now serialises local builds with flock and runs
them under `nice` with capped workers, so several agents building at
once queue instead of piling up daemons and compile workers and taking
the machine down. With CI/GITHUB_ACTIONS set it execs Gradle directly,
leaving CI untouched. Also shorten the Gradle daemon idle timeout so
idle daemons are reaped sooner.

AGENTS.md

@@ -14,6 +14,7 @@ Everything comes from the Nix flake devshell; do not install toolchains by hand.
 - The default devshell provides JDK 21, Gradle 9, the Android SDK (platform 37, build-tools 37.0.0, platform-tools 37.0.1) and nushell.
 - The integration harness needs real servers, so it lives in a separate `nix develop .#integration` shell (navidrome, gonic, lms, php, Nextcloud with the Music app, sqlite). Keeping them out of the default shell avoids pulling that closure into everyday builds.
 - Build with the devshell `gradle`, not `./gradlew`. CI uses the nix Gradle pinned by `flake.lock`; the wrapper is kept only for people without Nix.
+- The devshell `gradle` is a wrapper: outside CI it takes a lock (`flock /tmp/subtracks-gradle.lock`) and runs under `nice` with capped workers, so several agents building at once queue instead of exhausting the box. With `CI`/`GITHUB_ACTIONS` set (i.e. CI) it execs Gradle directly, unchanged.
 
 ## Commands
 

flake.nix

@@ -40,9 +40,17 @@
           };
           androidSdk = android.androidsdk;
 
+          gradle_wrapper = pkgs.writeShellScriptBin "gradle" ''
+            if [ -n "''${CI:-}''${GITHUB_ACTIONS:-}" ]; then
+              exec ${pkgs.gradle_9}/bin/gradle "$@"
+            fi
+            exec ${pkgs.util-linux}/bin/flock /tmp/subtracks-gradle.lock \
+              ${pkgs.coreutils}/bin/nice -n 10 ${pkgs.gradle_9}/bin/gradle --max-workers=4 "$@"
+          '';
+
           basePackages = [
             pkgs.jdk21
-            pkgs.gradle_9
+            gradle_wrapper
             pkgs.git
             pkgs.curl
             pkgs.unzip

gradle.properties

@@ -3,6 +3,7 @@ org.gradle.parallel=true
 org.gradle.caching=true
 org.gradle.configuration-cache=true
 org.gradle.configuration-cache.problems=warn
+org.gradle.daemon.idletimeout=600000
 
 android.useAndroidX=true
 android.nonTransitiveRClass=true