fix: harden cross-platform paths and child-process handling

Replace process.env.HOME fallbacks with os.homedir() in dashboard usage
probes and the hermes plugin profile resolver so unset HOME no longer
yields literal "~" paths. Skip POSIX process-group semantics on Windows
in engine/merger and dashboard-tui's pgrep-based vitest killer. Add
shell: true to npx spawns in CLI skills/extension so .cmd shims resolve
on Windows, and route test:build-exe through cross-env.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/package.json

@@ -45,7 +45,7 @@
     "build:exe:all": "bun run build.ts --all",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --silent=passed-only --reporter=dot",
-    "test:build-exe": "FUSION_TEST_BUILD_EXE=1 vitest run --config vitest.build-exe.config.ts --silent=passed-only --reporter=dot"
+    "test:build-exe": "cross-env FUSION_TEST_BUILD_EXE=1 vitest run --config vitest.build-exe.config.ts --silent=passed-only --reporter=dot"
   },
   "dependencies": {
     "@mariozechner/pi-ai": "^0.70.0",
@@ -82,6 +82,7 @@
     "@types/node": "^22.0.0",
     "@types/react": "^19.0.0",
     "@vitest/coverage-v8": "^3.1.0",
+    "cross-env": "^7.0.0",
     "ink-testing-library": "^4.0.0",
     "tsup": "^8.5.1",
     "tsx": "^4.19.0",