docs: rewrite the README in english for the reader it is aimed at

[Madeuss]

Point 3 of the internationalisation plan, after #12.

Who this is for

An engineer doing a technical screen spends five to fifteen minutes: README,
directory tree, one or two files. A recruiter spends thirty seconds on the first
paragraph. Neither reads Portuguese. Everything behind that first paragraph —
the ADRs, the solver, the isolation boundary — may as well not exist.

What changed

• Leads with the difficulty, not the feature list. "The chat is the easy
  part" is the sentence that decides whether someone keeps reading.
• Shows real CLI output, captured from make case SEED=42 LOCALE=en, not a
  description of it. Six dossiers with six different fact counts demonstrates
  context isolation better than a paragraph asserting it.
• A "what is hard here" section that hands the reader the two files worth
  opening — the solver that takes Case and never CaseWithSolution, and the
  domain where the solution is a separate entity — instead of making them guess
  from the tree.
• A diagram of the boundary: the never edge from Solution to NPC context
  is the whole security argument in one line.
• An honest status line. Phase 1 of 8, no NPCs yet, not playable. A README
  that oversells is worse than one in the wrong language.
• Roadmap as a record of what exists, with a note that LangGraph and Next.js
  arrive with the phases that need them — they are not in the repository today
  and the README should not imply they are.

Repository description and topics set too, since that is what appears in search
and on the profile.

Not here

A terminal GIF. The code block is real output and works everywhere, including
in a text-mode reader; a recording can replace it later without changing the
surrounding copy.

The Portuguese design documents stay Portuguese. They are the documents almost
nobody opens, and translating them is two days better spent elsewhere — the ADRs
are the ones worth translating, and that is the next PR.