Show link to created PR after Implementation step completes

[Neokil]

Problem

After the Implementation step completes and a PR is created, the UI shows no link to it. The user has to leave AutoPR and navigate to GitHub (or another VCS) manually to find the PR. This is a friction point that breaks the review flow, especially when managing multiple tickets.

Solution

1. Define a convention for PR URL output — e.g. the implementation script writes the PR URL to a well-known artifact file such as .autopr/runs/<uuid>/artifacts/pr_url.txt.
2. After the implementation state completes, the UI reads this file (via the existing artifact API) and renders a prominent "View PR" link.
3. If no PR URL artifact is present, the UI falls back gracefully with no broken state.