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

CONTRIBUTING.md

@@ -63,7 +63,7 @@ If English isn't your first language, don't worry! We value your contributions r
 
 ```bash
 # Clone the repository
-git clone https://github.com/code-yeongyu/oh-my-opencode.git
+git clone https://github.com/code-yeongyu/oh-my-openagent.git
 cd oh-my-opencode
 
 # Install dependencies (bun only - never use npm/yarn)