Feature Request: Support branch specification when registering plugin marketplaces

Summary

When registering a plugin marketplace via /plugin, there is currently no way to specify a particular branch of a repository. It would be useful to support branch specification syntax so users can point to a specific branch without needing workarounds.

Current Behavior

The /plugin command accepts the following formats:

• owner/repo (GitHub shorthand)
• git@github.com:owner/repo.git (SSH)
• https://example.com/marketplace.json (HTTPS URL)
• ./path/to/marketplace (local path)

All remote repository formats default to the repository's default branch. There is no way to specify an alternative branch.

Proposed Behavior

Support branch specification syntax when adding a marketplace source. For example:

owner/repo#branch-name
git@github.com:owner/repo.git#branch-name
https://github.com/owner/repo#branch-name

This would allow users to register plugins from feature branches, development branches, or version-specific branches directly.

Use Cases

• Testing plugin changes: Plugin developers working on a feature branch want to test their changes before merging to main.
• Version pinning: Users may want to pin to a specific release branch (e.g., owner/repo#v2).
• Staging/preview: Teams can share work-in-progress plugins from non-default branches for review.

Current Workarounds

1. Clone the repo locally, checkout the desired branch, and register using a local path.
2. Fork the repo and set the desired branch as the default branch, then register the fork.

Both workarounds add unnecessary friction compared to native branch support.