Add droid custom partner agent (#406)

* Add droid custom partner agent

Add a new Droid CLI assistant agent to the partners collection that provides installation guidance, usage examples, and automation patterns for the Droid CLI, with emphasis on droid exec for CI/CD and non-interactive automation.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

* Address PR #406 comments on droid agent

- Add model field to frontmatter
- Fix escaped pipe in Quick Reference table (remove unnecessary backslash from markdown table code cell)

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

---------

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>
Co-authored-by: Aaron Powell <me@aaron-powell.com>

README.md

@@ -24,7 +24,7 @@ Discover our curated collections of prompts, instructions, and chat modes organi
 | Name | Description | Items | Tags |
 | ---- | ----------- | ----- | ---- |
 | [Awesome Copilot](collections/awesome-copilot.md) | Meta prompts that help you discover and generate curated GitHub Copilot chat modes, collections, instructions, prompts, and agents. | 6 items | github-copilot, discovery, meta, prompt-engineering, agents |
-| [Partners](collections/partners.md) | Custom agents that have been created by GitHub partners | 17 items | devops, security, database, cloud, infrastructure, observability, feature-flags, cicd, migration, performance |
+| [Partners](collections/partners.md) | Custom agents that have been created by GitHub partners | 18 items | devops, security, database, cloud, infrastructure, observability, feature-flags, cicd, migration, performance |
 
 
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