fix(install): setup wizard is a subcommand now, not a --setup flag

Both installers still invoked `ostp --setup` / `ostp.exe --setup` to launch
the first-run wizard on a fresh install. The CLI's subcommand refactor
(2026-07-08, "Refactor CLI to subcommands") turned `setup` into
`Commands::Setup { .. }` with no top-level `--setup` flag left in Args at
all, so every fresh install has hit "error: unexpected argument '--setup'
found" and dropped the user out of the installer instead of the wizard.
Verified `ostp setup --help` parses correctly with the fix.

scripts/install.ps1

@@ -138,5 +138,5 @@ Write-Host "No configuration found. Launching setup wizard..."
 Write-Host ""
 
 Push-Location $InstallDir
-& .\ostp.exe --setup
+& .\ostp.exe setup
 Pop-Location

scripts/install.sh

@@ -238,4 +238,4 @@ echo "No configuration found. Launching setup wizard..."
 echo ""
 
 cd "$INSTALL_DIR"
-exec ./ostp --setup --config "$CONFIG_FILE"
+exec ./ostp setup --config "$CONFIG_FILE"