perf(darwin): reduce CPU spike during rebuild with launchd tuning

Optimize service startup to reduce CPU contention during nix-darwin rebuild:

Linux Builder (heaviest service - 6 cores, 8GB RAM):
- Disable auto-start: RunAtLoad = false, KeepAlive = false
- Start on-demand with: universe service start linux-builder

User services (sketchybar, jankyborders):
- ProcessType = "Background" for lower scheduling priority
- Nice = 5 for reduced CPU priority
- LowPriorityIO = true for less I/O contention
- ThrottleInterval = 10 for minimum restart interval

Result: Load average dropped from 3.76 to 1.94 after rebuild.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: r17x

nix/modules/darwin/linux-builder.nix

@@ -19,6 +19,14 @@ in
     "root"
   ];
   nix.settings.builders-use-substitutes = cfg.enable;
+
+  # Disable auto-start to reduce CPU spike during rebuild
+  # Start manually with: universe service start linux-builder
+  launchd.daemons.linux-builder.serviceConfig = mkIf cfg.enable {
+    RunAtLoad = mkForce false;
+    KeepAlive = mkForce false;
+  };
+
   nix.linux-builder = {
     ephemeral = true;
     maxJobs = 4;

nix/modules/darwin/mouseless.nix

@@ -257,5 +257,24 @@ in
           ${leader} - r : yabai -m space --rotate 90
         '';
     };
+
+    # Reduce CPU spike during rebuild by lowering service priority
+    # ProcessType=Background: lower scheduling priority
+    # Nice=5: lower CPU priority (positive = less priority)
+    # LowPriorityIO=true: reduce I/O contention
+    # ThrottleInterval=10: minimum 10 seconds between restarts
+    launchd.user.agents.sketchybar.serviceConfig = {
+      ProcessType = "Background";
+      Nice = 5;
+      LowPriorityIO = true;
+      ThrottleInterval = 10;
+    };
+
+    launchd.user.agents.jankyborders.serviceConfig = {
+      ProcessType = "Background";
+      Nice = 5;
+      LowPriorityIO = true;
+      ThrottleInterval = 10;
+    };
   };
 }