Trim AGENTS.md to focus on actionable content

Remove bloated sections (full feature inventory, port tables, env var
tables, label categories, health checks) that waste context window.
Keep only what an AI agent needs: build/test commands, architecture
essentials, coding conventions, and key gotchas.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Tharsanan1