feat: GitHub App Integration — deeper GitHub sync (PR linking, issue sync, installations)

[OneStepAt4time]

GitHub App Integration — Deeper GitHub Sync

Why

Currently Aegis has webhook delivery and basic GitHub API access. Multica has a full GitHub App with PR linking, issue sync, and installation flow. Deeper GitHub integration means agents can create PRs, link issues, and participate in the GitHub workflow natively.

Concept

• Register a GitHub App that users install on their repos
• PR Linking: agent sessions automatically link to PRs they create
• Issue Sync: GitHub issues can be assigned to agents, status syncs back
• Installation Flow: user installs app, selects repos, Aegis receives webhooks

Features

1. GitHub App registration and installation flow
2. PR creation detection and linking to sessions
3. Issue assignment to agents (bi-directional sync)
4. Status updates posted back to GitHub (comments, labels)
5. Webhook receiver for GitHub events (push, PR, issue)

Acceptance Criteria

• GitHub App manifest and registration
• Installation callback handler
• PR linking to sessions
• Issue-to-agent assignment sync
• Status comments on GitHub
• Tests
• npm run gate passes

Reference

Competitive analysis: references/multica-competitive-analysis.md §2 (GitHub integration)