fix(cli): verify Bun.which() path exists before using on Windows

Bun.which('bun') on Windows CI returns a non-existent path
(C:\Users\runneradmin\.bun\bin\bun.exe), causing all integration
tests to fail. Add existsSync() check to fallback to process.execPath
when the resolved path doesn't exist.

Also fix version-command.test.ts to use process.execPath instead of
hardcoded 'bun' string for cross-platform compatibility.

Fixes 19 failing integration tests on Windows platform.

cli/test/helpers/run-cli.ts

@@ -1,6 +1,9 @@
+import { existsSync } from 'node:fs'
+
 export async function runCli(args: string[], env: Record<string, string> = {}) {
-  // Use Bun.which() to find bun in PATH, fallback to current executable
-  const bunPath = await Bun.which('bun') || process.execPath
+  // Use Bun.which() to find bun in PATH, but verify it exists
+  const whichBun = await Bun.which('bun')
+  const bunPath = (whichBun && existsSync(whichBun)) ? whichBun : process.execPath
 
   const proc = Bun.spawn({
     cmd: [bunPath, 'src/index.ts', ...args],

cli/test/integration/version-command.test.ts

@@ -26,8 +26,11 @@ describe('version command', () => {
     const dir = await mkdtemp(join(tmpdir(), 'skillhub-node-build-'))
     const outfile = join(dir, 'index.js')
     await writeFile(join(dir, 'package.json'), JSON.stringify({ type: 'module' }))
+
+    // Use process.execPath to ensure we use the current Bun executable
+    const bunPath = process.execPath
     const build = Bun.spawn({
-      cmd: ['bun', 'build', 'src/index.ts', '--target=node', `--outfile=${outfile}`],
+      cmd: [bunPath, 'build', 'src/index.ts', '--target=node', `--outfile=${outfile}`],
       cwd: new URL('../../', import.meta.url).pathname,
       stdout: 'pipe',
       stderr: 'pipe'