docs: update all GitHub URLs from oh-my-opencode to oh-my-openagent

The GitHub repository was renamed from oh-my-opencode to oh-my-openagent,
but all documentation, scripts, and source code references still pointed
to the old repository name. This caused confusion for users who saw
'oh-my-opencode' in docs but a different repo name on GitHub.

Updated all references across:
- README files (en, ko, ja, zh-cn, ru)
- CONTRIBUTING.md
- docs/ (installation, overview, configuration, etc.)
- Source code (schema URLs, GitHub API calls, issue links)
- Test snapshots

The npm package name remains 'oh-my-opencode' (unchanged).

Fixes: https://x.com/Dhruv14588676/status/2031216617762468348

src/cli/model-fallback.test.ts

@@ -517,7 +517,7 @@ describe("generateModelConfig", () => {
 
       // #then should include correct schema URL
       expect(result.$schema).toBe(
-        "https://raw.githubusercontent.com/code-yeongyu/oh-my-opencode/dev/assets/oh-my-opencode.schema.json"
+        "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-opencode.schema.json"
       )
     })
   })