docs(readme): user-focused rewrite

Leads with install via mise (github backend, since ubi is deprecated and gets removed in mise 2027.1.0), shows a quick-taste workflow, summarizes the design promise, and pushes build-from-source to the bottom for contributors. Drops em dashes throughout, adds a few platform emojis on the release-target list.

Verified install path: ``` $ mise x github:tuist/fabrik@latest – fabrik –version fabrik 0.1.0 ```

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