feat: add LangSmith observability and Discord alerts

[BenjaminGrabow]

Summary

Adds LangSmith observability configuration for LangGraph runs and Discord webhook alerting.

What changed

• Added typed LangSmith config parsing from environment variables
• Passed run name, tags, metadata, and thread IDs into LangGraph invocations for LangSmith traces
• Added Discord webhook notifier with rich embeds
• Added notification environment parsing
• Extended .env.example with LangSmith and Discord variables
• Added observability and notification docs
• Added tests for LangSmith config and Discord notifier behavior

Tests

• pnpm run check passes locally
• pnpm run replay passes locally

Notes

LangSmith tracing stays disabled by default. Discord is inert unless DISCORD_WEBHOOK_URL is configured.