docs: frame the README around the behavior contract and lane

[IdoGol24]

Summary

Stacked on #232. Reframes the README around what the system does rather than how it is built: engineers should not need to know how their system is implemented, only how it behaves.

• Leads with the behavior contract (responses, stored state, runtime effects) using the existing subscription example; the Playwright spec becomes a footnote.
• Adds the behavior lane (CODE → VERIFY → SHIP) with the verification-loop diagram, relabeled: behavior contract, real system, findings as held / failed / not observed.
• Relabels the system-boundary diagram to what to run / how to act on it / how to observe it.
• Moves Playwright and Feature files into a short "Writing contracts today" section.
• Marks setup in another repository as unavailable until the npm release.

Scope and links

• Closes #N: none
• Actual publication/source branch: chore/readme-behavior-contract
• Target base: docs/readme-alpha-status (docs: reposition the README and add alpha status #232)
• Delivery phase: Post-Alpha documentation

Type of change

• Docs / tooling only

Validation and evidence

Command	Result
npx prettier@3.9.9 --check README.md	pass
relative link and anchor check on README.md	pass, 13 links
run commands	unchanged from #232, not rerun

• Known limitations: docs/alpha-status.md is unchanged; the six other diagrams in docs/assets/readme/ stay unreferenced.

Security impact

• Trust boundaries affected: none
• No secrets or private traces are included in code, logs, or artifacts

Independent review evidence

• Not requested; draft for discussion.

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