Implement GitHub webhook handler for auto-closing issues on PR merge #13

Problem

The Integrations settings page shows webhook setup instructions and a copy-able endpoint URL, but the actual webhook handler for GitHub push/PR events is not fully implemented. Branch names referencing issue IDs (e.g., ENG-12) should auto-transition issues.

Current State

• Webhook URL exists at /github-webhook on the Convex HTTP endpoint
• Webhook setup instructions are shown in Settings > Integrations
• Connected repos are tracked in the connectedRepos table
• But no event handler parses incoming GitHub payloads

Acceptance Criteria

• PR opened with issue ID in branch/title -> issue status changes to in_progress
• PR merged -> issue status changes to done
• Activity log entry created for each auto-transition
• Only processes events from connected repos
• Slack notification sent if webhook is configured