### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When a skill/plugin is registered with a `git-subdir` source, the Skill Hub UI builds the clickable source link by hardcoding the branch to `main`:

https://github.com/BerriAI/litellm/blob/main/ui/litellm-dashboard/src/components/skill_hub_table_columns.tsx#L82

```js url = src.path ? `${src.url}/tree/main/${src.path}` : src.url; ```

The repository's actual default branch is never consulted. For a repo whose default branch is `master` (or any non-`main` branch), the generated link is wrong and 404s on GitHub, e.g.:

- Generated: `https://github.com/org/repo/tree/main/my-skill` → **404** - Correct: `https://github.com/org/repo/tree/master/my-skill`

The stored `source` object (`{source: "git-subdir", url, path}`) has no branch/ref field, so there's no way for a user to correct it either.

### Steps to Reproduce

1. Have a GitHub repo whose default branch is `master` (not `main`), with a skill in a subdirectory (e.g. `my-skill/SKILL.md`). 2. Register it: `POST /claude-code/plugins` with `{"name":"my-skill","source":{"source":"git-subdir","url"…