Redesign the public README and documentation

[Nickfost]

Summary

Turns repository Markdown into a clear public documentation system:

• redesigns the root README as a GitHub landing page for new users;
• explains the problem, intended audience, architecture, trust model, project contract, status, and roadmap;
• adds visual architecture and growth diagrams;
• adds search-friendly terminology for self-hosted runners, Docker CI, Proxmox, test sharding, and distributed workers;
• adds a task-oriented documentation index and glossary;
• replaces the opaque docs/adr presentation with plain-language controller and configuration guides;
• documents how a public source repository can use private fleet CI and deployment through a protected private delivery repository;
• links operators directly to host, project, migration, security, maintenance, and pilot procedures.

This PR changes documentation only. It does not rename the repository or live installation, modify the pilot controller, or claim that setup.sh is already implemented.

Closes #26
Relates to #21, #24, and #27.

[Nickfost]

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