docs: add Docker environment variables table to README

[avoidwork]

Summary

Adds a comprehensive Docker Environment Variables section () with all configuration variables organized into categorized tables:

• Essential —
• Optional — Providers — OpenAI, OpenRouter settings
• Optional — Tools — Fal.ai, Exa, Firecrawl, Tavily, Parallel, SearXNG, Bing, Custom search
• Optional — Sandbox — paths, timeouts, permissions, memory limits
• Optional — Memory — persistent storage directories
• Optional — Telemetry — OpenTelemetry exporter settings
• Optional — Schedules — cron scheduler settings
• Optional — TUI — terminal UI settings
• Optional — Persistence — storage backend settings
• Alternative — inline env var references in config.yaml (recommended approach for containers)

Changes

• Simplified the Quick Start Docker example to show only essential variables
• Moved environment variables from Development to Docker section
• Added full variable table mapping to config.yaml keys with defaults
• Cleaned up redundant Docker recommendation note at top of file
• Updated all cross-references to point to the new section

Docker Users

This makes it much easier for Docker users to know exactly which -e flags they need. Instead of guessing or digging through config files, they get a clear reference table with defaults and descriptions.

Security

No changes to security behavior — this is documentation only. Also notes that inline ${VAR} references in config.yaml are the recommended approach for containers since secrets won't appear in docker inspect output.