docs: rewrite README.md sentence by sentence against the prose rules
Every sentence is written for a newcomer to read once: one idea, subject and verb first, under about 25 words, bullets instead of comma lists, no tables, no project history. The order is what the extension is, what it talks to, how to build and load it, the settings, the design, then development and tests. Every statement was checked against the code on next; the disagreements with the old text are listed in the PR. Part of #672. Model: fable-5-1
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