feat(aura-cli): --help/-h print usage to stdout and exit 0

`aura --help` and `aura -h` are a newcomer's reflex first command but
fell through to the catch-all unknown-subcommand arm: exit 2, empty
stdout, error text on stderr. The conventional help affordance returning
the error path is a C22 first-contact friction (milestone fieldtest).

Add a `Some("--help") | Some("-h")` arm before the catch-all that prints
the usage to stdout and returns normally (exit 0). The `_ =>` error path
is untouched, so an unknown subcommand still prints `aura: usage: aura
run` to stderr and exits 2 -- pinned by the negative-preservation
assertion in the RED test (07ba20a). Verified: cli_run.rs all green
(help/no-args/run), clippy --all-targets -D warnings clean.

Minimal slice per the tdd handoff: the stdout help literal ("usage: aura
run") and the stderr error literal ("aura: usage: aura run") are left as
two distinct user-facing strings rather than factored into a shared
usage constant -- a single usage source is a deliberate non-goal here,
left for a future cycle if one wants it.

closes #20

crates/aura-cli/src/main.rs

@@ -115,6 +115,7 @@ fn main() {
     let mut args = std::env::args().skip(1);
     match args.next().as_deref() {
         Some("run") => println!("{}", run_sample().to_json()),
+        Some("--help") | Some("-h") => println!("usage: aura run"),
         _ => {
             eprintln!("aura: usage: aura run");
             std::process::exit(2);