build: dendritic flake, devShell and crate skeleton

src/main.rs

@@ -0,0 +1,12 @@
+// Modules land before the CLI uses them; Task 11 removes this.
+#![allow(dead_code)]
+
+use clap::Parser;
+
+#[derive(Parser)]
+#[command(version, about = "macOS-style screenshots for niri")]
+struct Cli {}
+
+fn main() {
+    Cli::parse();
+}