docs: comprehensive README — all features, providers, API, sandbox, custom tools

New sections:
- CLI Reference: all commands and flags with examples
- Providers & Models: Deepseek, OpenAI, self-hosted (Ollama, vLLM)
- Thinking Levels: --thinking flag deep dive with provider mapping
- Sandbox: full security model (6 hardening dimensions)
- Built-in Tools: shell tool schema and agent workflow
- Custom Tools: Tool interface with full example
- Programmatic API: Go library usage with Config reference table
- Configuration: env vars, defaults, fallback order
- System Prompt: customization examples
- Development: test suite, coverage table, contributing guide

Kept all existing content — Install, Quick Start, Design, Architecture, Security, License.

Author: molty3000