PR selector for work items with multiple linked PRs

[Flo0806]

Problem

When an issue has multiple linked PRs, the Changes tab silently shows only contributions[0]. There's no way to switch between linked PRs.

Solution

Add a PR selector (dropdown or tabs) when contributions.length > 1, allowing users to pick which PR's diff to view. The work item sidebar (#203) would be a natural place for this selector.

Current behavior

• activePullNumber and activePullHeadSha use contributions[0]
• Changes tab shows only the first linked PR
• No visual indicator that other PRs exist

Affected files

• app/pages/repos/[owner]/[repo]/work-items/[id].vue — computed getters
• app/components/work-item/WorkItemChanges.vue — would need to react to selection