feat(04-03): add cross-platform build script and build:all npm script

- Create scripts/build-all-platforms.mjs that detects current platform, invokes electron-builder via build-desktop.mjs, verifies artifacts in build/release/, and reports remaining platforms
- Add "build:all" script entry to package.json for D-07 cross-platform build validation

package.json

@@ -1,6 +1,6 @@
 {
   "name": "agent-company",
-  "version": "0.4.53",
+  "version": "0.4.54",
   "private": true,
   "type": "module",
   "description": "Cross-platform Electron desktop control plane for local-first AI agent companies.",
@@ -12,6 +12,7 @@
     "build:verify": "electron-vite build",
     "build": "electron-vite build && node scripts/build-desktop.mjs",
     "build:dir": "electron-vite build && node scripts/build-desktop.mjs --dir",
+    "build:all": "node scripts/build-all-platforms.mjs",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "typecheck": "tsc --noEmit",