ci(docs): Remove agentic codebase documentation workflows (#5655)

Remove the agentic CI workflows and generated artifacts introduced for
auto-generating codebase documentation.

This reverts the work from #5649 and #5654, which added GitHub Actions
workflows to generate and refresh docs using an AI agent. The generated
docs files (`BUILD_PLAN.md`, `_meta/manifest.json`,
`_meta/style-guide.md`) and associated action lock files are also
removed.

The `.gitattributes` file (which was added as part of that workflow
setup) is removed as well.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dingsdax