docs: restructure README around user workflow

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing
+
+## Building
+
+```
+make build
+```
+
+Builds the `zen` binary with version + commit SHA stamped via ldflags. Or manually:
+
+```
+go build -o zen .
+```
+
+Verify your build:
+
+```
+zen version
+```
+
+## Testing
+
+```
+make test
+```
+
+## Architecture
+
+See [docs/architecture.md](docs/architecture.md) for the daemon design, source-of-truth model, worktree naming conventions, and source tree layout.