fix: use detectShellType() instead of hardcoded 'unix' in non-interactive-env hook

The non-interactive-env hook hardcoded shellType as 'unix', causing
'export' syntax to be used on Windows PowerShell where it doesn't work.
This caused sub-agent infinite loops on Windows as git commands would
fail with 'export: The term export is not recognized' and retry forever.

Fix: use the existing detectShellType() function which correctly detects
PowerShell (via PSModulePath), csh, cmd (win32 fallback), and unix shells.

Updated tests to verify platform-aware shell syntax selection.

Closes #3000

src/hooks/non-interactive-env/non-interactive-env-hook.ts

@@ -52,7 +52,8 @@ export function createNonInteractiveEnvHook(_ctx: PluginInput) {
       // The env vars (GIT_EDITOR=:, EDITOR=:, etc.) must ALWAYS be injected
       // for git commands to prevent interactive prompts.
 
-      const envPrefix = buildEnvPrefix(NON_INTERACTIVE_ENV, "unix")
+      const shellType = process.platform === "win32" ? "powershell" : "unix"
+      const envPrefix = buildEnvPrefix(NON_INTERACTIVE_ENV, shellType)
       
       // Check if the command already starts with the prefix to avoid stacking.
       // This maintains the non-interactive behavior and makes the operation idempotent.