fix: prepend YAML frontmatter to Cursor .mdc files (#1699)

* fix: prepend YAML frontmatter to Cursor .mdc files for auto-inclusion

Cursor IDE requires YAML frontmatter with `alwaysApply: true` in .mdc
rule files for them to be automatically loaded. Without this frontmatter,
users must manually configure glob patterns for the rules to take effect.

This fix adds frontmatter generation to both the bash and PowerShell
update-agent-context scripts, handling three scenarios:
- New .mdc file creation (frontmatter prepended after template processing)
- Existing .mdc file update without frontmatter (frontmatter added)
- Existing .mdc file with frontmatter (no duplication)

Closes #669

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

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: address Copilot review suggestions

- Handle CRLF line endings in frontmatter detection (grep '^---' instead
  of '^---$')
- Fix double blank line after frontmatter in PowerShell New-AgentFile
- Remove unused tempfile import from tests
- Add encoding="utf-8" to all open() calls for cross-platform safety

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

scripts/powershell/update-agent-context.ps1

@@ -258,6 +258,12 @@ function New-AgentFile {
     # Convert literal \n sequences introduced by Escape to real newlines
     $content = $content -replace '\\n',[Environment]::NewLine
 
+    # Prepend Cursor frontmatter for .mdc files so rules are auto-included
+    if ($TargetFile -match '\.mdc$') {
+        $frontmatter = @('---','description: Project Development Guidelines','globs: ["**/*"]','alwaysApply: true','---','') -join [Environment]::NewLine
+        $content = $frontmatter + $content
+    }
+
     $parent = Split-Path -Parent $TargetFile
     if (-not (Test-Path $parent)) { New-Item -ItemType Directory -Path $parent | Out-Null }
     Set-Content -LiteralPath $TargetFile -Value $content -NoNewline -Encoding utf8
@@ -334,6 +340,12 @@ function Update-ExistingAgentFile {
         $newTechEntries | ForEach-Object { $output.Add($_) }
     }
 
+    # Ensure Cursor .mdc files have YAML frontmatter for auto-inclusion
+    if ($TargetFile -match '\.mdc$' -and $output.Count -gt 0 -and $output[0] -ne '---') {
+        $frontmatter = @('---','description: Project Development Guidelines','globs: ["**/*"]','alwaysApply: true','---','')
+        $output.InsertRange(0, $frontmatter)
+    }
+
     Set-Content -LiteralPath $TargetFile -Value ($output -join [Environment]::NewLine) -Encoding utf8
     return $true
 }