feat: Add GitHub trigger, platform registry, and multi-platform tutorials

- Implement GitHub webhook trigger with full support for:
  - Push, PR, Issues, Workflow, CheckRun events
  - HMAC-SHA256 signature verification
  - PR reviews, comments, status checks via API
  - Repository/event/user filtering

- Add PlatformRegistry for dynamic plugin loading:
  - Factory-based platform instantiation
  - Runtime platform registration
  - PlatformCapabilities for feature discovery

- Create comprehensive user tutorials:
  - Telegram bot for ops/sre (runbooks, incidents, approvals)
  - WhatsApp bot for on-call workflows
  - GitHub automation (PR reviews, deployments, releases)

- Add v1alpha1 AgentFlow examples:
  - github/pr-security-review.yaml
  - github/deploy-on-merge.yaml
  - telegram/runbook-executor.yaml
  - telegram/incident-manager.yaml
  - whatsapp/oncall-alert.yaml
  - whatsapp/deployment-approval.yaml

- Create developer documentation for building custom triggers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: gouravjshah

crates/aof-triggers/src/lib.rs

@@ -45,7 +45,11 @@ pub use platforms::{
     DiscordConfig, DiscordPlatform,
     TelegramConfig, TelegramPlatform,
     WhatsAppConfig, WhatsAppPlatform,
+    GitHubConfig, GitHubPlatform,
     TypedPlatformConfig,
+    // Platform registry for extensibility
+    PlatformRegistry, PlatformFactory,
+    PlatformCapabilities, get_platform_capabilities,
 };
 
 /// Core trait for trigger platforms