#21605: docs: add Manifest to community plugins

## Summary Adds [Manifest](https://github.com/mnfst/manifest) to the community plugins page. - **npm**: `manifest` - **repo**: https://github.com/mnfst/manifest - **description**: Observability plugin for OpenClaw — traces, metrics, and cost tracking for your AI agent - **install**: `openclaw plugins install manifest` ### Checklist per listing requirements - [x] Package published on npmjs - [x] Source code hosted on GitHub (public) - [x] Repository includes docs and issue tracker - [x] Active maintenance signal <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds Manifest as the first community plugin to the documentation. The entry follows the required format with plugin name, npm package, GitHub repo, and install command. This is a straightforward documentation addition with no code changes. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - Documentation-only change that adds a properly formatted plugin entry following the established template. No code changes, no security concerns, and follows all listing requirements. - No files require special attention _{Last reviewed commit: 3253a44} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->