feat: Batching + rate limiting for webhook notifications

[nexus-marbell]

Summary

Add batching and rate limiting to the webhook notification pipeline to handle high-volume repos gracefully. When many events arrive in quick succession (e.g., a CI run creating multiple check events, or a batch of issues being filed), collapse them into summary messages instead of flooding agents.

RFC: #177 (GitHub Webhook to Swarm Notification Bridge)

Design

Cooldown / Batching

• Track events per repo within a rolling time window
• If 5+ events from the same repo arrive within 60 seconds, batch them into a single summary message
• Summary format: [GitHub] 7 events on finml-sage/repo in the last 60s: 3 issue comments, 2 PR reviews, 2 issues opened
• After the batch is sent, reset the counter for that repo
• Use an in-memory buffer (dict of repo -> event list with timestamps)

Rate Limiting

• Global rate limit: maximum N webhook-triggered swarm messages per minute (configurable, default 30)
• Per-repo rate limit: maximum M messages per minute per repo (configurable, default 10)
• When rate limited, queue events and send a summary when the window resets
• Prevents webhook abuse (accidental or intentional flood of events)

Configuration

• Sensible defaults that work without configuration
• Optional env var overrides for tuning:
  • GITHUB_WEBHOOK_BATCH_THRESHOLD (default: 5 events)
  • GITHUB_WEBHOOK_BATCH_WINDOW (default: 60 seconds)
  • GITHUB_WEBHOOK_RATE_LIMIT (default: 30 messages/minute global)

Implementation Notes

• Use asyncio primitives for the batching timer (not threads)
• The batch buffer should be cleaned up periodically to prevent memory leaks from repos that go quiet
• Batch summary should still include the [GitHub] prefix for consistency
• Individual high-priority events (e.g., new external contributor's first issue) could bypass batching in a future enhancement

Acceptance Criteria

• Events from the same repo within the batch window are collapsed into a summary
• Batch threshold is configurable (default 5 events)
• Batch window is configurable (default 60 seconds)
• Global rate limit prevents more than N messages per minute
• Per-repo rate limit prevents flood from a single repo
• Batch summaries include event type counts (e.g., "3 issue comments, 2 PRs")
• Batch summaries include the [GitHub] prefix
• In-memory buffer is cleaned up for inactive repos
• Uses asyncio primitives (no threads)
• Unit tests cover: batching trigger, batch summary format, rate limit enforcement, buffer cleanup

Dependencies

Depends on #180 (event formatting -- batching wraps around the formatted messages).