Milestone Burndown Template link GitHub milestones to standup goals

[csharpfritz]

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)

[github-actions]

📋 Assigned to Linus (Backend Dev)

Issue: #75 — Milestone Burndown Template link GitHub milestones to standup goals

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