Issue Backlog View tree view of squad-labeled GitHub issues

[csharpfritz]

Description

Add a new tree view showing all squad:{member} labeled issues from GitHub, grouped by member and priority. Shifts SquadUI from a read-only observer to an active project management tool.

Approach

• New SquadBacklogTreeProvider tree data provider
• Group by member (squad:{member} labels), then by priority label
• Show issue title, number, state, and assignee
• Click to open issue in browser or show details in a webview
• Requires GitHub API pagination + caching for rate limits
• Extends existing GitHubIssuesService

Acceptance Criteria

• New 'Backlog' tree view appears in the Squad sidebar
• Issues grouped by squad member
• Priority sub-grouping within each member
• Click opens issue details
• Refresh button to re-fetch from GitHub
• Handles rate limiting gracefully

[github-actions]

📋 Assigned to Rusty (Extension Dev)

Issue: #71 — Issue Backlog View tree view of squad-labeled GitHub issues

Rusty will pick this up in the next Copilot session.

For Copilot coding agent: If enabled, this issue will be worked automatically.
Otherwise, start a Copilot session and say:
Rusty, work on issue #71