Add the project skeleton and the approved design

Cargo.toml with the release profile and lints, the toolchain pinned to
Rust 1.90.0, a build script that records the git commit for
`cumulus --version`, and rustfmt settings and cargo aliases (`cargo lint`,
`cargo linux`) as in skyshift. DESIGN.md holds the design approved after
the question rounds; README.md has build instructions and the planned keys.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/main.rs

@@ -0,0 +1,37 @@
+//! cumulus: brainstorm in the terminal.
+//!
+//! This file only reads the command line. Everything else lives in the
+//! library: `src/lib.rs` lists the modules.
+
+use std::process::ExitCode;
+
+const USAGE: &str = "\
+cumulus: brainstorm in the terminal
+
+Usage: cumulus
+
+Options:
+  -h, --help       show this help
+  -V, --version    show the version";
+
+fn main() -> ExitCode {
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    match args.first().map(String::as_str) {
+        None => {
+            println!("cumulus is being built, one stage at a time: see DESIGN.md.");
+            ExitCode::SUCCESS
+        }
+        Some("-h" | "--help") => {
+            println!("{USAGE}");
+            ExitCode::SUCCESS
+        }
+        Some("-V" | "--version") => {
+            println!("{}", cumulus::version());
+            ExitCode::SUCCESS
+        }
+        Some(other) => {
+            eprintln!("cumulus: unknown option {other}\n\n{USAGE}");
+            ExitCode::FAILURE
+        }
+    }
+}