fix(mcpb): add Windows support via platform_overrides and run.cmd

The previous manifest hardcoded /bin/sh which fails on Windows. Fix by:
- Adding mcpb/run.cmd: Windows batch wrapper that searches common uvx
  install locations (%USERPROFILE%\.local\bin, %LOCALAPPDATA%\uv\bin,
  %USERPROFILE%\.cargo\bin) before falling back to PATH, with a clear
  error message pointing to winget/PowerShell install instructions
- Adding platform_overrides.win32 in mcp_config to use cmd /c run.cmd
  on Windows while macOS/Linux continue using /bin/sh run.sh
- Expanding compatibility.platforms to include win32

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

mcpb/run.cmd

@@ -0,0 +1,27 @@
+@echo off
+REM Locate uvx — tries common Windows install locations first, then PATH
+
+for %%P in (
+    "%USERPROFILE%\.local\bin\uvx.exe"
+    "%USERPROFILE%\.cargo\bin\uvx.exe"
+    "%LOCALAPPDATA%\uv\bin\uvx.exe"
+) do (
+    if exist %%P (
+        %%P nextcloud-mcp-server run --transport stdio
+        exit /b %errorlevel%
+    )
+)
+
+REM Fall back to uvx on PATH if found
+where uvx >nul 2>&1
+if %errorlevel% equ 0 (
+    uvx nextcloud-mcp-server run --transport stdio
+    exit /b %errorlevel%
+)
+
+REM uvx not found — print actionable error
+echo Error: 'uvx' was not found in any expected location. >&2
+echo Install uv (which provides uvx) from: https://docs.astral.sh/uv/getting-started/installation/ >&2
+echo   Windows: winget install astral-sh.uv >&2
+echo   or:      powershell -c "irm https://astral.sh/uv/install.ps1 | iex" >&2
+exit /b 1