opencode: add JSON schema reference to config.json output

- Automatically include "$schema": "https://opencode.ai/config.json" in the generated
  opencode config.json file
- Update documentation to mention the added schema property
- Add corresponding "$schema" entry in test config.json file for validation purposes

Closes #7506

tests/modules/programs/opencode/config.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "https://opencode.ai/config.json",
   "autoshare": false,
   "autoupdate": true,
   "model": "anthropic/claude-sonnet-4-20250514",