docs: add BundleDex badge to README

[mcclawd]

Adds the BundleDex badge to show this repository is indexed in the BundleDex OKF directory. BundleDex is the go-to directory for Open Knowledge Format bundles used by AI agents.

The BundleDex listing at https://bundledex.net/bundles/agent-knowledge/ has been updated:

• Category: Changed from 'Example Datasets' to 'AI Agent Skills'
• Description: Updated to reflect the skills.sh/plugin install workflow
• The listing now correctly describes the project's actual workflow

If you'd like any further adjustments, please let me know!

[mcclawd]

Hi @stjbrown! 👋

Following up on your feedback on the previous PR (#2). The BundleDex listing at https://bundledex.net/bundles/agent-knowledge/ has been updated:

1. Category: Changed from 'Example Datasets' → 'AI Agent Skills' — matching the kb-* agent skill family nature of the project
2. Description: Updated to: 'Give your coding agent a portable knowledge base — install via skills.sh or as a Claude Code plugin. The kb- agent skill family maintains, queries, and health-checks OKF knowledge bundles.'*
3. The listing now accurately reflects the skills.sh/plugin workflow

I've created this new PR to add the BundleDex badge. Please take a look and let me know if any further adjustments are needed!