fix(windows): set windowsHide on remaining child spawns to stop console flash (#1092) (#1104)

On Windows, a black console (conhost) window flashed briefly when CodeGraph
ran as a background MCP server. Several child spawns were missing
`windowsHide: true`, so Windows created a visible console for the child:

- scripts/npm-shim.js — launching the bundled runtime (every server start /
  daemon-idle reconnect) and the self-heal `tar` extraction of a missing
  platform bundle.
- src/reasoning/login.ts — the detached `cmd /c start` browser open.
- src/upgrade/index.ts — package-manager spawn (console-attached, so no flash
  in practice, but set for uniformity: every child spawn now hides).

The daemon spawn (#411) and all git execFileSync sites already set it; this
closes the remaining gaps. Adds an all-platforms source guard to
__tests__/npm-shim.test.ts asserting every spawn in the shim sets windowsHide.

Closes #1092

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

__tests__/npm-shim.test.ts

@@ -71,6 +71,23 @@ function runShim(pkgDir: string, args: string[], env: Record<string, string>) {
   });
 }
 
+// Static source guard (all platforms): every child spawn in the shim must set
+// windowsHide, or a console (conhost) window flashes when the shim runs as a
+// background MCP server on Windows (issue #1092). windowsHide is a Windows-only
+// spawn behavior that can't be observed from these POSIX-only subprocess tests,
+// so we assert it at the source level instead — and this also catches any new
+// spawn site added to the shim later.
+describe('npm-shim windowsHide (#1092)', () => {
+  it('sets windowsHide: true on every spawn in the shim', () => {
+    const src = fs.readFileSync(SHIM_SRC, 'utf8');
+    const spawnLines = src.split('\n').filter((l) => /\.spawn(Sync)?\(/.test(l));
+    expect(spawnLines.length).toBeGreaterThan(0); // guard against a false pass if the calls move
+    for (const line of spawnLines) {
+      expect(line, `spawn without windowsHide: ${line.trim()}`).toContain('windowsHide: true');
+    }
+  });
+});
+
 describe.skipIf(isWindows)('npm-shim launcher', () => {
   it('runs the installed optional-dependency bundle without any download', async () => {
     const pkg = makePkg();