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.ts

@@ -20,7 +20,7 @@ export type { GeneratedOmoConfig } from "./model-fallback-types"
 const ZAI_MODEL = "zai-coding-plan/glm-4.7"
 
 const ULTIMATE_FALLBACK = "opencode/glm-4.7-free"
-const SCHEMA_URL = "https://raw.githubusercontent.com/code-yeongyu/oh-my-opencode/dev/assets/oh-my-opencode.schema.json"
+const SCHEMA_URL = "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-opencode.schema.json"