refactor: simplify compile.ts to single-target and add per-platform compile scripts

- Rewrite compile.ts from 112 to 66 lines: single target with auto-detect host, remove multi-target batch logic
- Add compile:linux/mac/win scripts to server, root, and turbo configs
- Wire desktop dist:* to depend on matching server compile:* (avoid unnecessary cross-platform compilation)
- Update AGENTS.md docs across root, server, and desktop

apps/server/turbo.json

@@ -8,6 +8,15 @@
     },
     "compile": {
       "outputs": ["out/**"]
+    },
+    "compile:linux": {
+      "outputs": ["out/**"]
+    },
+    "compile:mac": {
+      "outputs": ["out/**"]
+    },
+    "compile:win": {
+      "outputs": ["out/**"]
     }
   }
 }