docs(project): disclose the agentic development model of the Rust client #52

Outcome

Make the Rust client README explain that it is part of a Codex-driven agentic
reimplementation and improvement of human-developed Atrinik Classic, built to
present a world whose pixel art, maps, stories, music, and sound effects are
made by people.

Human-written code contributions remain welcome. “Agentic” describes the
project's primary current software-development workflow, not a requirement that
every commit or contributor use an agent.

Scope

• Add a short, prominent Development model or equivalent section near the
  README introduction.
• State that the replacement client is fresh MIT-licensed Rust/SDL3 code
  developed primarily through Codex workflows under maintainer direction,
  review, provenance controls, tests, and repository validation.
• Describe it as a reimplementation and improvement of the Classic client and
  experience, not a mechanical C translation or source port.
• Link the replacement roadmap and canonical project authorship statement.
• State that the presented game content and media remain human-created external
  inputs with their exact authors, upstream credit, licenses, and notices.
• Update CONTRIBUTING.md only as needed to make direct human code
  contributions explicitly welcome under the same clean-room, review, and
  validation rules.

Acceptance criteria

• The first README screenful identifies the Rust client as part of
  Atrinik's agentic next-generation reimplementation.
• The wording credits Codex-driven development without claiming every line
  or commit is agent-written and without diminishing human contributors.
• Human-authored pixel art/audio/world content is clearly separated from
  code authorship and the client's MIT license.
• Existing clean-room/GPL and asset-provenance boundaries are unchanged or
  strengthened.
• tools/validate.sh and git diff --check pass.

The implementation direction remains tracked by
atrinik/atrinik#168.