Clean up README prose

[akiselev]

What changed

• mark the project explicitly as experimental
• replace the LLM-agent tagline and rationale with a description of persistent DAP sessions and shell automation
• remove marketing labels such as zero-friction, rich, and LLM-friendly
• normalize headings, option lists, adapter status text, and contribution instructions

Why

The README should explain the debugger architecture and command surface directly. Structured output remains documented as a general automation capability rather than being framed around a particular class of client.

Validation

Documentation-only change. The branch changes only README.md; command names and examples are preserved.