Rewrite the README in plain English

[pedro-pscunha]

What changed

The README is rewritten in plain English (the simple-english skill, pragmatic mode). It now
follows the heading order and the glossary that the three guideme SDKs share: one word for one
meaning ("question", "answer", "option", "level", "rubric", "unsure", "setting", "state"), short
sentences, no semicolons outside the rendered rubric, and conditions before commands.

New heading order: Install, Quick start, Questions, When the model is not sure, Examples and
counterexamples, Several questions in one request, The receipt, Errors, Retries and timeouts,
Testing your code, Observability, Configuration, Sync and async, Lower layers, Other SDKs,
Development, License.

• Every Python block is unchanged, so tests/typing/readme.py still mirrors the first example
  and the other blocks it copies.
• Every name in guideme.__all__ is still inside code (tests/test_surface.py).
• Lower layers keeps the second-tier semver promise (guideme.api, guideme.api.client,
  guideme.policy).
• Other SDKs has three rows. The TypeScript row links the repository only and says
  "@guideme/sdk (not yet on npm)".
• All links stay absolute, because the README is the PyPI long description.
• The retry and timeout rules move out of Errors and Configuration into their own section.

What moved where

• The measured 0.75 / 0.17 criteria case, why no timeout is resent, why a transport refuses a
  timeout, and the two Usage classes: already in docs/design.md. The README links them.
• Why Client sits in guideme.api.client: new bullet in docs/design.md, Sharp edges.
• The sdist test command: CONTRIBUTING.md, The loop.
• The checker set, the test style, the live tests and the hooks: already in AGENTS.md.

Other files

• CONTRIBUTING.md: the tier list drops guideme.question.Question, which AGENTS.md moved
  to the top-level list, and names guideme.api.client.
• docs/contract.md: one reference renamed. The timeout divergence is now in the README's
  Retries and timeouts section. No new prose.
• CHANGELOG.md: an Unreleased → Documentation note. No version bump. The PyPI 0.2.0 page
  keeps the old README until the next release.

Length

README: 568 lines before, 503 after the review fixes. Code blocks: 127 lines before, 121 after.
The shared target was "at most about 320". The review accepted about 470 as the honest length
of one file, because the code blocks are tied to the typing mirror and the rubric contract.

How I verified it

• Every behavioural claim was checked against src/guideme/**: builder defaults and refusals
  (guide.py), retry and resend rules (api/client.py), rubric rules (enums.py,
  question.py), state serialization (_json.py), receipt and error fields.
• A claim ledger maps each factual sentence of the old README to its new line, a docs/
  line, or a reason it was dropped. Three rows are dropped, all framing text with no user fact.
• rg -n -i readme over the repository: every hit that names a heading or a block was checked,
  and docs/contract.md was updated.
• The pre-push hook ran the full mise run check: ruff, pyright (0 errors), pylint (10.00),
  pytest (230 passed), build and audit.

Review fixes (second commit)

An opus review found 0 Critical, 4 Major and 17 Minor items. All are applied:

• The quick start defines the state, the three constructors, the level order and "unsure".
• The policy layers list the setter for each kind of question again.
• The score value states its scale, from the TypeSafe API docs.
• The base_url row says that guideme follows redirects.
• The error rows now list the missing TransportError and ConfigError cases.
• The typing notes moved to Lower layers.
• Testing points to the retry rules in place of repeating them.
• The spec-vendoring lines left Other SDKs.