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>

src/shell-command-projection.cts

@@ -427,6 +427,7 @@ export function execGit(args: string[], opts: { cwd?: string; env?: Record<strin
     encoding: 'utf-8',
     stdio: 'pipe',
     timeout: opts.timeout ?? 10_000,
+    windowsHide: true,
   });
   return _spawnResult(result, 'git');
 }
@@ -438,6 +439,7 @@ export function execNpm(args: string[], opts: { cwd?: string; timeout?: number }
     encoding: 'utf-8',
     stdio: ['ignore', 'pipe', 'pipe'],
     timeout: opts.timeout ?? 15_000,
+    windowsHide: true,
   });
   return _spawnResult(result, 'npm');
 }
@@ -449,6 +451,7 @@ export function execTool(program: string, args: string[], opts: { cwd?: string;
     encoding: 'utf-8',
     stdio: 'pipe',
     timeout: opts.timeout ?? 30_000,
+    windowsHide: true,
   });
   return _spawnResult(result, program);
 }