feat(onboarding): surface-aware download strip and model-library intro fact

[quiet-node]

Overview

Polishes the first-run experience around the built-in engine. The onboarding intro now tells users they can run any open-source model, and the download status strip adapts its copy to where it is shown so the call to action always matches what the user can actually do at that moment.

What changed

• New onboarding fact, "Run any open-source AI model," introducing the in-app model library: download from Hugging Face in a click, swap models in Settings, with a fit check for the user's Mac. The intro lead-in changes from "Five" to "A few" quick tips so the count stays accurate now that there are six facts.
• DownloadStatusStrip takes a surface prop ('askbar' | 'onboarding') in place of the previous alternate boolean. The ready line reads "Send your first message!" on the ask bar and "Hit Get Started to start chatting!" during onboarding, where the compose surface is not yet available.
• CLAUDE.md drops the duplicated built-in engine and model-library internals (these now live in docs/models-and-providers.md) and gains a short "Since v0.15" summary of the local-first shift.

How it works

surface is a required, explicit discriminator, so every call site declares its context rather than relying on a defaulted flag. The ask-bar surface keeps the alternating "safe to close" hint; onboarding shows a steady label and a ready line pointing at the Get Started button. Behavior is preserved exactly: alternate was false during onboarding and true on the ask bar, which maps to surface === 'askbar'.

Testing

• bun run typecheck, bun run lint:frontend, and the Prettier check are clean.
• bun run test:coverage: 96 files, 2129 tests pass, 100% coverage held across lines, branches, functions, and statements.

Note: the change is frontend and docs only; no backend files are touched, so the Rust gates are unchanged from main and confirmed in CI.