Fix #2405: Add template parameter to GitHub issue URL (#2407)

When using 'pack buildpack register', the GitHub URL was missing
the 'template=add-buildpack.md' parameter, causing GitHub to redirect
to a template selection page and drop all pre-filled parameters.

This fix adds the template parameter to ensure the correct issue
template is used and all parameters are preserved.

🤖 Generated with [Claude Code](https://claude.ai/code)

Signed-off-by: Juan Bustamante <bustamantejj@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: jjbustamante

pkg/client/register_buildpack.go

@@ -68,6 +68,7 @@ func (c *Client) RegisterBuildpack(ctx context.Context, opts RegisterBuildpackOp
 		params := url.Values{}
 		params.Add("title", issue.Title)
 		params.Add("body", issue.Body)
+		params.Add("template", "add-buildpack.md")
 		issueURL.RawQuery = params.Encode()
 
 		c.logger.Debugf("Open URL in browser: %s", issueURL)