fix(#685): set windowsHide on all Windows child-process spawns (#688)

* fix(#685): set windowsHide on all Windows child-process spawns

A visible "gsd-core" console window flashed on Windows whenever a gsd-core
child process spawned without `windowsHide: true`. The most visible offenders
fire on every SessionStart / `/clear` (execNpm's `shell:true` npm view via the
update-check worker) and on every Edit/Write/MultiEdit in a worktree (the
worktree-path guard's git probe).

Add `windowsHide: true` to every external-binary spawn in the runtime source:
- hooks/gsd-context-monitor.js (record-session spawn)
- hooks/gsd-worktree-path-guard.js (SPAWNOPT)
- hooks/gsd-workflow-guard.js (git branch --show-current)
- src/shell-command-projection.cts (execGit / execNpm / execTool)
- src/check-command-router.cts (git log execFileSync)
- src/roadmap-upgrade.cts (git status/rev-parse/reset/clean execSync)

gsd-check-update.js already had it (the precedent). probeTty's tty call is
POSIX-only and intentionally untouched. Adds a regression test that asserts
each site plus a repo-wide completeness guard so a future external-binary
spawn that omits windowsHide fails CI. No behavior change off-Windows.

Closes #685

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#685): set changeset pr number to 688

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

hooks/gsd-context-monitor.js

@@ -150,7 +150,7 @@ process.stdin.on('end', () => {
         spawn(
           process.execPath,
           [gsdTools, 'state', 'record-session', '--stopped-at', stoppedAt],
-          { cwd, detached: true, stdio: 'ignore' }
+          { cwd, detached: true, stdio: 'ignore', windowsHide: true }
         ).unref();
         warnData.criticalRecorded = true;
         // Persist the sentinel so subsequent debounce cycles don't re-fire

hooks/gsd-workflow-guard.js

@@ -61,6 +61,7 @@ function currentBranch(cwd) {
     cwd,
     encoding: 'utf8',
     stdio: ['ignore', 'pipe', 'ignore'],
+    windowsHide: true,
   });
   if (result.status !== 0) return '';
   return result.stdout.trim();

hooks/gsd-worktree-path-guard.js

@@ -18,7 +18,7 @@ const fs = require('fs');
 const path = require('path');
 const { spawnSync } = require('child_process');
 
-const SPAWNOPT = { encoding: 'utf8', stdio: ['ignore', 'pipe', 'ignore'], timeout: 2000 };
+const SPAWNOPT = { encoding: 'utf8', stdio: ['ignore', 'pipe', 'ignore'], timeout: 2000, windowsHide: true };
 
 function git(args, cwd) {
   return spawnSync('git', args, { ...SPAWNOPT, cwd });