docs: improve GitHub discoverability

[Azakeen]

Summary

• sharpen the README positioning around local-first MCP context for AI coding agents
• add clearer navigation, live project badges, a star call to action, and a real Spectra-generated architecture map
• align Cargo package descriptions and keywords with the project’s GitHub positioning

Why

The existing documentation was technically complete, but the first screen did not immediately communicate the product category, supported agents, or core value. This update makes the repository easier to understand and discover while keeping the claims factual and the prototype status prominent.

Impact

Documentation and package metadata only. Runtime behavior is unchanged.

Validation

• cargo fmt --check
• cargo metadata --locked --no-deps --format-version 1
• cargo test --workspace --locked
• git diff --check

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