feat: add Volta Node version manager support

Added support for Volta (https://docs.volta.sh/), a Node version manager
for Windows/macOS/Linux. This resolves issue #295 where OpenCode installed
via Volta was not being detected.

Changes:
- Added ~/.volta/bin to Windows path expansion in getExpandedEnv()
- Added Volta paths to Windows OpenCode detection in probeWindowsPaths()
- Volta's shim directory (~/.volta/bin) is now searched when detecting tools

Users with OpenCode installed via Volta will now see it automatically
detected in the Wizard and available for group chat moderator selection.

src/main/agents/path-prober.ts

@@ -87,6 +87,8 @@ export function getExpandedEnv(): NodeJS.ProcessEnv {
 			// Node Version Manager for Windows (nvm4w) - OpenCode commonly installed here
 			'C:\\nvm4w\\nodejs',
 			path.join(home, 'nvm4w', 'nodejs'),
+			// Volta - Node version manager for Windows/macOS/Linux (installs shims to .volta/bin)
+			path.join(home, '.volta', 'bin'),
 			// Scoop package manager (OpenCode, other tools)
 			path.join(home, 'scoop', 'shims'),
 			path.join(home, 'scoop', 'apps', 'opencode', 'current'),
@@ -255,6 +257,9 @@ function getWindowsKnownPaths(binaryName: string): string[] {
 			// Scoop installation (recommended for OpenCode)
 			path.join(home, 'scoop', 'shims', 'opencode.exe'),
 			path.join(home, 'scoop', 'apps', 'opencode', 'current', 'opencode.exe'),
+			// Volta - Node version manager (OpenCode commonly installed via Volta)
+			path.join(home, '.volta', 'bin', 'opencode'),
+			path.join(home, '.volta', 'bin', 'opencode.cmd'),
 			// Chocolatey installation
 			path.join(
 				process.env.ChocolateyInstall || 'C:\\ProgramData\\chocolatey',