fix: skip remote URIs when resolving MCP server cwd (#10844)

When the extension runs in the Local Extension Host on Windows while
connected to a remote workspace (SSH, Dev Container, etc.),
getWorkspaceDirs() returns vscode-remote:// URIs. These cannot be used
as a cwd for child_process.spawn(), causing "spawn cmd.exe ENOENT"
when starting MCP stdio servers.

Fall back to the user's home directory for non-file:// URIs so the
spawn has a valid local working directory.

Fixes #10842

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