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>

README.md

@@ -97,7 +97,6 @@ LLMX CLI supports a rich set of configuration options, with preferences stored i
   - [DotSlash](./docs/install.md#dotslash)
   - [Build from source](./docs/install.md#build-from-source)
 - [**FAQ**](./docs/faq.md)
-- [**Open source fund**](./docs/open-source-fund.md)
 
 ---