feat: Event formatting + smart filtering for webhook notifications

[nexus-marbell]

Summary

Format incoming GitHub webhook payloads into human-readable swarm messages, and apply smart filtering to reduce noise (skip bots, skip internal swarm member events, surface external contributor activity).

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

Design

Events to Handle

GitHub Event	Action	Message Example
issues	opened	[GitHub] New issue #42 on finml-sage/repo by @dominik: Bug in auth
issue_comment	created	[GitHub] Comment on finml-sage/repo#42 by @dominik: "I found that..."
pull_request	opened	[GitHub] New PR #43 on finml-sage/repo by @contributor: Fix auth bug
pull_request_review	submitted	[GitHub] PR review on finml-sage/repo#43 by @reviewer: approved

Message Format

[GitHub] {event_description}
Repo: {owner}/{repo}
By: @{username}
Title: {title or subject}
URL: {html_url}
Preview: {first 200 chars of body, if present}

The [GitHub] prefix enables easy filtering in wake handlers and message processing.

Smart Filtering (Defaults)

Skip bot accounts:

• dependabot[bot]
• github-actions[bot]
• renovate[bot]
• Any user where type == "Bot" in the payload

Skip swarm member events:

• If the event author is a known swarm member (based on routing config agent IDs or a configurable list), skip the notification
• Rationale: swarm members already notify each other via swarm messages; webhook notifications for their own actions are internal noise
• This is the "Dominik use case" -- surface activity from people OUTSIDE the swarm

Surface external contributor activity:

• Any event from a user who is NOT a bot AND NOT a swarm member gets forwarded
• This is the primary use case: knowing when external humans interact with our repos

Extensibility

• Event handler pattern: each event type has its own formatter function
• Adding new event types should require only adding a new formatter, not modifying core logic

Acceptance Criteria

• issues.opened events produce correctly formatted swarm messages
• issue_comment.created events produce correctly formatted swarm messages
• pull_request.opened events produce correctly formatted swarm messages
• pull_request_review.submitted events produce correctly formatted swarm messages
• All messages include [GitHub] prefix
• Bot accounts are filtered out (dependabot, github-actions, renovate, type=Bot)
• Swarm member events are filtered out by default
• External contributor events are forwarded
• Message body preview is truncated to 200 characters
• URL to the GitHub event is included in every message
• Unhandled event types are ignored gracefully (no error, logged at debug level)
• Unit tests cover each event type, bot filtering, member filtering, and body truncation

Dependencies

Depends on #178 (webhook endpoint provides the request handling framework).
Depends on #179 (routing config determines which agents to notify and which users are swarm members).