chore: Remove OpenAI branding and references

Update project branding to reflect community fork status:

- Changed welcome message from "OpenAI's command-line coding agent"
  to "your command-line coding agent"
- Updated system prompt from "led by OpenAI" to "community project"
- Removed broken developers.openai.com/llmx documentation URLs
- Updated Windows setup instructions with correct npm package name
- Removed OpenAI CLA (not applicable to community fork)
- Removed OpenAI open source fund documentation
- Updated config docs to remove managed configuration references
- Changed npm package reference from @openai/llmx to @valknarthing/llmx

These changes complete the rebranding from OpenAI's project to an
independent community fork while maintaining proper attribution to
the original project in README.md.

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

Co-Authored-By: Claude <noreply@anthropic.com>

llmx-rs/tui/src/onboarding/welcome.rs

@@ -69,7 +69,7 @@ impl WidgetRef for &WelcomeWidget {
             "  ".into(),
             "Welcome to ".into(),
             "LLMX".bold(),
-            ", OpenAI's command-line coding agent".into(),
+            ", your command-line coding agent".into(),
         ]));
 
         Paragraph::new(lines)