Add groupBy support to githubPullRequests.queries (like githubIssues.queries) #9095

Feature Request

Problem

githubIssues.queries supports a groupBy property with \"repository\" and \"milestone\" values, allowing issues to be grouped by repo in the tree view. githubPullRequests.queries has no equivalent — it only accepts label and query.

This makes cross-repo PR queries hard to navigate. For example, a query like:

{
  "label": "All My Open PRs",
  "query": "is:open is:pr author:@me"
}

returns PRs from multiple repositories but displays them as a flat list with no indication of which repo each PR belongs to.

Proposed Solution

Add groupBy support to githubPullRequests.queries, matching the schema already present on the issues side:

{
  "label": "All My Open PRs",
  "query": "is:open is:pr author:@me",
  "groupBy": ["repository"]
}

This would group PR results under owner/repo sub-nodes in the tree, making cross-repo queries usable.

Prior Art

• githubIssues.queries already supports groupBy: ["repository", "milestone"]
• Issue Include repo name in PR tree view #5453 addressed repo name display for checked-out PRs, but the query list view was not covered

Workaround

Currently the only workaround is to create one query entry per repository, which doesn't scale.