fix: convert shellenv array to string for PowerShell Invoke-Expression

The issue was that PowerShell's & operator returns an array of output lines,
but Invoke-Expression requires a single string.

Error: 'Cannot convert System.Object[] to the type System.String required by parameter Command'

Fix: Use -join "`n" to convert the array to a newline-delimited string.

This allows Invoke-Expression to properly evaluate the shellenv output.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: timvw

.github/workflows/ci.yml

@@ -464,7 +464,7 @@ jobs:
 
             # Load shellenv
             $env:WORKTREE_ROOT = $WORKTREE_ROOT
-            Invoke-Expression (& $WT_BIN shellenv)
+            Invoke-Expression ((& $WT_BIN shellenv) -join "`n")
 
             # Test checkout
             Write-Output "Testing: wt checkout test-branch"