fix(doctor): add explicit exit 0 on success (PowerShell)

doctor.ps1 already exited 1 on failure but relied on the implicit process
exit code on success, which a stale $LASTEXITCODE from an earlier native
call (e.g. codex --version) could turn into a false failure in CI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/doctor.ps1

@@ -66,3 +66,4 @@ foreach ($path in @('shared/rules','shared/skills','shared/agents','shared/hooks
 
 if ($fail) { exit 1 }
 Write-Output 'PASS doctor'
+exit 0