[AG-10] Add branch-per-task workflow with PR creation

[0xtechdean]

Summary

• Add git-service.ts for branch/PR management via GitHub API
• Auto-create branches when tasks start, PRs when tasks complete
• Add GitHub webhook for PR merge → task done status updates
• New "PR Review" column in Kanban board with PR badges

Changes

File	Description
src/git-service.ts	New service for branch/PR operations
src/taskdb.ts	Add branch, prUrl, prNumber, prStatus fields
src/controllers/tasks.controller.ts	Branch creation + PR creation logic
src/index.ts	GitHub webhook handler
src/orchestrator.ts	Add branch to AgentContext
public/index.html	PR Review column, PR badges, branch display

Configuration Required

Set these environment variables:

• GITHUB_TOKEN - GitHub personal access token with repo scope
• GITHUB_OWNER - Repository owner (default: Othentic-Labs)
• GITHUB_REPO - Repository name (default: ai-team)
• GITHUB_WEBHOOK_SECRET - (Optional) Webhook signature verification

Acceptance Criteria

• Agent creates branch when starting task
• Branch follows naming convention: task/{id}-{slug}
• Agent creates PR when completing task
• PR description includes task context
• PR link is stored on task record
• Task status updates when PR is merged
• Dashboard shows PR status on tasks
• Branches are cleaned up after merge

Test Plan

1. Create a task with an owner
2. Move task to "Ready" → should create branch
3. Agent completes → should create PR
4. Merge PR → task moves to "Done"

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