Milestone Burndown Template link GitHub milestones to standup goals

## Description
Enhance the standup report to link burndown charts directly to GitHub milestones. Allow users to select a milestone and see progress toward it, connecting squad metrics to actual release tracking.

## Approach
- Fetch milestones from GitHub API via `GitHubIssuesService`
- Add milestone selector to standup report webview
- Burndown chart shows issues open vs closed against milestone due date
- Reuse existing burndown charting infrastructure

## Acceptance Criteria
- [ ] Standup report shows milestone selector dropdown
- [ ] Burndown chart tracks selected milestone's issue progress
- [ ] Shows milestone due date and completion percentage
- [ ] Works with fork-aware issue fetching (upstream milestones)