docs: split the README into a landing page and CONTRIBUTING

The README was two documents in one, and neither reader was served by
the other's half. It opened on a feature list, then spent its second
half on Go versions, WebKitGTK packages and `make` targets — while its
install table named a `darwin-universal.app.zip` and a
`windows-amd64.exe` that nothing has ever produced, and its header
claimed Windows and never mentioned Android, which is the one platform
with a published, self-updating channel.

So this is a correctness pass as much as a friendliness one. The README
now answers a user's questions only: what the app is, three screenshots
from the seeded fixture library so anyone can retake them, the four
formats, one install section per channel that names what is actually
published, first run, where the data lives, and pointers out. The
version-restart note is linked to the two documents that own it rather
than copied, because a copy is a second thing to keep true.

CONTRIBUTING.md takes the technical half: prerequisites, the system
libraries, the build and codegen commands, which verification tier a
change demands, the tracker workflow, the commit grammar and the style
rules. CLAUDE.md is unchanged apart from one paragraph naming the split
— it was already the deep reference both of the others point at, and
stays the only one of the three that explains why a shape is what it is.

Closes #50

CLAUDE.md

@@ -6,6 +6,22 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 YellowJacket is a cross-platform desktop music player built with Go (backend) and TypeScript/Lit (frontend), using the Wails framework to bridge them. It supports MP3, FLAC, OGG Vorbis, and WAV playback.
 
+**The three prose documents are split by reader, not by topic** (#50).
+`README.md` is the landing page and answers *a user's* questions only —
+what it does, which channel installs it on which platform, where its
+data lives — with three screenshots in `docs/images/`, captured from the
+fixture library (`make sandbox-seed NAME=default` → `make dev-headless
+SEED=default`) so they can be retaken by anyone. `CONTRIBUTING.md` holds
+what used to be the second half of that README — prerequisites, the
+system libraries, the build and codegen commands, which verification
+tier a change demands, the tracker workflow and the commit grammar. This
+file stays the deep reference both of them point at, and is the only one
+of the three that explains *why* a shape is what it is. A fact that
+belongs to a user goes in one place; the packaging channels keep their
+own documents (`packaging/*/README.md`, `docs/android-release.md`) and
+are linked rather than summarised, because a version-restart note copied
+into the README is a second copy to keep true.
+
 ## Issues
 
 **The tracker is the source of truth for what is wanted and what is