fix(opencode): use correct global config path ~/.config/opencode (#117)

The OpenCode installer was writing to ~/.opencode but OpenCode expects
global configuration at ~/.config/opencode per XDG Base Directory spec.

Fixes:
- src/commands/install.ts: Change default output from ~/.opencode to
  ~/.config/opencode
- src/targets/opencode.ts: Recognize "opencode" basename (not just
  ".opencode") for direct writes without nesting

Closes #114

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

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

src/commands/install.ts

@@ -175,7 +175,9 @@ function resolveOutputRoot(value: unknown): string {
     const expanded = expandHome(String(value).trim())
     return path.resolve(expanded)
   }
-  return path.join(os.homedir(), ".opencode")
+  // OpenCode global config lives at ~/.config/opencode per XDG spec
+  // See: https://opencode.ai/docs/config/
+  return path.join(os.homedir(), ".config", "opencode")
 }
 
 async function resolveGitHubPluginPath(pluginName: string): Promise<ResolvedPluginPath> {