feat(credentials): add scoped credential helpers

scripts/commands/add-credentials.ps1

@@ -0,0 +1,39 @@
+[CmdletBinding()]
+param(
+    [ValidateSet('Codex', 'Claude', 'Both')][string]$Client,
+    [string]$Key,
+    [switch]$ValueFromStdin,
+    [string]$HomePath = [Environment]::GetFolderPath('UserProfile')
+)
+$ErrorActionPreference = 'Stop'
+$root = Split-Path (Split-Path $PSScriptRoot -Parent) -Parent
+. (Join-Path $root 'scripts/lib/install-targets.ps1')
+$selectedClient = $Client
+$selectedKey = $Key
+. (Join-Path $root 'scripts/lib/credentials.ps1')
+$Client = $selectedClient
+$Key = $selectedKey
+
+if (-not $Client) { $Client = Read-InstallClient -Client $Client }
+if (-not $Key) { $Key = Read-Host 'Credential key' }
+Assert-CredentialKey $Key
+if (-not [System.IO.Path]::IsPathRooted($HomePath)) { throw 'HomePath must be absolute.' }
+if ($ValueFromStdin) {
+    $text = [Console]::In.ReadToEnd().TrimEnd("`r", "`n")
+    $Value = ConvertTo-SecureString $text -AsPlainText -Force
+    $text = $null
+} else {
+    $Value = Read-Host 'Credential value' -AsSecureString
+}
+
+$bin = Join-Path $HomePath '.my-ai-configuration/bin'
+New-Item -ItemType Directory -Path $bin -Force | Out-Null
+foreach ($source in @('credentials.ps1', 'with-credential.sh')) {
+    $destination = Join-Path $bin $(if ($source -eq 'credentials.ps1') { 'with-credential.ps1' } else { $source })
+    if (-not (Test-Path -LiteralPath $destination)) { Copy-Item (Join-Path $root "scripts/lib/$source") $destination }
+}
+foreach ($target in $(if ($Client -eq 'Both') { @('Codex', 'Claude') } else { @($Client) })) {
+    Set-ManagedCredential $target $Key $Value
+    Write-Output "Credential stored for $($target.ToLowerInvariant())."
+}
+$Value = $null