Support active workspace folder for Pi terminals

[0xff26b9a8]

Summary

• Resolve Pi terminal cwd from the workspace folder containing the active editor in multi-root windows
• Apply the same active-workspace cwd selection to the terminal profile, Open with File context, and @pi chat RPC
• Support VS Code-style variables in pi-vscode.path, including ${userHome}, ${workspaceFolder}, ${workspaceFolder:name}, ${env:NAME}, and ${/}
• Add unit coverage for active workspace selection, fallback behavior, and configured path variable expansion

Verification

• pnpm run fmt
• pnpm run typecheck
• pnpm exec vitest run test/resolve.test.ts