docs: rewrite README — problem-first, value-first, half the words

[arielshulman29]

Rewrites the README around the story instead of the mechanics (596 → ~190 lines):

1. Problem-first opening — the web was built for humans; a new era of AI agents begins, and most sites are invisible to them.
2. One-time setup — install (npm/pnpm/yarn) + npx ax init; from then on every build generates the artifacts automatically. Also corrects the install guidance to a regular dependency: @ora-ai/ax/middleware is imported by user middleware.ts and ships in the production bundle (devDependencies only worked by accident of bundling, and breaks prune-before-build flows).
3. Value-first feature section — six benefit→feature entries (agents.md · markdown twins · AI Catalog + llms.txt · auth detection/declared auth/auth.md · agent-aware 404 + wayfinding · JSON-LD), each honest about mechanism (detect vs. scaffold vs. generate).
4. The report — the coding-agent loop, mapped to Ora's checks.
5. How it works — prebuild/postbuild map + runtime middleware, placed after the value sections.
6. Contract sections kept in trimmed form: status note, supported matrix (out-of-scope column folded into its intro sentence), ax init, the ax.config example (now showcasing declared auth), repository layout. Design-posture section and deep-dive duplicates folded or cut.

Targets main — #25 (the features this text documents) is already merged.

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