docs: restructure README for the v0.15 built-in-engine launch

[quiet-node]

Overview

Rewrites the README around the v0.15 shift: Thuki now ships its own built-in llama.cpp inference engine as the default, with Ollama demoted to one of several supported providers. The page is tighter, more accurate, and leads with the new one-line installer.

Key changes

• Restructure. Sharpened hero value sentence, install moved up under the demo, "Why Thuki?" trimmed to four bullets, Features cut to the ones that earn a spot, dedicated Models & providers and Privacy sections.
• Install. curl -fsSL https://thuki.app/install.sh | sh is the recommended path (RSA-verified DMG, no Gatekeeper prompt and no manual xattr step). Manual DMG and build-from-source move into collapsibles; a production-build path (bun run build:all then open the bundle) is added.
• Providers. Built-in engine framed as the default; Ollama and a future OpenAI-compatible option grouped under "Other supported providers."
• Accuracy fixes carried into the app and docs: OCR copy now states it extracts text, not scenes, in the README, config/tips.ts, config/commandArtifacts.ts, and the regenerated docs/commands.md. The /think bullet is reframed as opt-in, off by default.
• Onboarding copy. The roadmap cap line is softened from an absolute ("All free. All local...") to "Local-first, always," and its comment no longer claims voice and meeting notes are paid cloud features (a not-yet-decided detail).
• /search roadmap note ("coming built-in, no Docker") propagated to the two sandbox/search-box service READMEs and pointed at tracking issue Ship first-class, out-of-box /search (no Docker setup) #257.
• Architecture & security folded back into a compact README collapsible, avoiding a thin standalone doc that would duplicate the subsystem docs.
• Founder note added below the roadmap.

Testing

• bun run test (frontend): 2176 pass.
• bun run typecheck, prettier --check, bun run build:frontend: clean.
• Frontend coverage is 100% on every changed file. The only sub-100% line is App.tsx:1296, a pre-existing local-only artifact that is green in CI. No Rust changed.

Note

Two links go live with the 0.15.0 landing-page work and are dead until then: thuki.app/install.sh (the one-line installer) and thuki.app/subscribe (founder-note email capture). This README should land together with that release, not before. The OpenAI-compatible provider is intentionally a future roadmap mention only; the openai kind stays gated in code.