feat: add GitHub webhook logic

[juanmahidalgo]

GitHub Webhook Integration

Summary

Adds a GitHub webhook listener that lets the agent respond to comments on issues and PRs directly from GitHub. When someone mentions @jarvis in a GitHub comment, the agent processes the request and replies as a GitHub comment — no Slack required.

What it does

• POST /github/webhook endpoint that receives GitHub webhook events
• Handles two event types:
  • issue_comment — comments on issues or PRs (top-level)
  • pull_request_review_comment — inline review comments on PR diffs
• Detects review requests (@jarvis review this PR) and uses Opus model for those
• Replies inline to review comment threads, or as top-level comments on issues/PRs
• Audit logs to Slack when LOG_CHANNEL_ID is configured

Security layers (in order)

Layer	What it does
Payload size limit	Rejects payloads > 1 MB
HMAC-SHA256 signature	Verifies X-Hub-Signature-256 against GITHUB_WEBHOOK_SECRET
Repo org check	Rejects events from repos outside the configured org
Repo allowlist	Optional — restricts to specific repos via GITHUB_ALLOWED_REPOS
Bot filter	Ignores comments from bots and from itself (anti-loop)
Org membership	Verifies sender is a member of the GitHub org via gh api (cached 1h)
Per-user rate limit	Max 10 requests per user per 5-minute window
Response truncation	Caps responses at 65K chars (GitHub comment limit)
Generic error messages	Never leaks internal errors to GitHub comments

New environment variables

Variable	Required	Default	Description
GITHUB_WEBHOOK_SECRET	Yes (to enable)	—	Webhook secret for signature verification. If not set, webhook handler is disabled.
GITHUB_ORG	No	decentraland	GitHub org for membership checks
GITHUB_ALLOWED_REPOS	No	—	Comma-separated repo allowlist. Empty = all org repos allowed.

Notes

• The GITHUB_TOKEN should have read:org scope for the membership check to work with private org members. Without it, only public members are recognized.
• The webhook handler uses its own AgentScheduler instance, sharing the same concurrency limit (MAX_CONCURRENT_AGENTS) — a spike in GitHub events won't starve Slack requests since they use separate schedulers.
• Feature is opt-in: if GITHUB_WEBHOOK_SECRET is not set, the handler is not initialized and the endpoint returns 503.