docs(readme): simplify getting started

What changed

This simplifies the project readme so the first path is installation and agent setup:

• Removed the header image and the separate integrations section.
• Added installation through mise using the current GitHub release.
• Added a coding harness configuration for the Model Context Protocol server, started through mise exec so global installs and project pins both work.
• Added a concrete Rust starter prompt that asks the harness to discover target kinds, fetch the Rust binary starter, write files, and build the target.
• Kept the script example as the secondary path for existing automation.

Why

The readme was doing too much before it reached the setup path. The new shape keeps the top of the file focused on installing Once and connecting it to a coding harness, which is the path someone needs before trying target discovery or project initialization.

Approach

The server example includes --allow-run because the documented Rust starter workflow asks the harness to build the generated target. It also starts Once through mise exec, which lets the same snippet work with a global Once install or a project-local mise.toml pin.

Impact

This is documentation-only. It changes the first-run guidance for readers and does not affect the command-line behavior or public APIs.

Validation

• git diff --check README.md
• Installed and ran the current release with mise x github:tuist/once@0.24.1 -- once --version, which reported once 0.24.1.
• Validated the documented Model Context Protocol path in a temporary project with Once 0.24.1 and Rust 1.96.0 pinned through mise. The harness-style session advertised once_query_example and once_build_target, fetched rust-binary-with-crate, wrote 7 starter files, and built apps/hello/hello successfully through once_build_target.

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