feat(vm): capture the payload's output into a host directory

The console is one stream, kernel and guest mixed, so a consumer that
needs the payload's stdout and stderr apart - rhema's sandbox seam
reports them as separate fields - could not get it. With capture set,
the run adds the directory as an internal rw share (guest path
/.ecrou-io, reserved) and the init wraps the payload in a redirection
group pointing at it: the streams land host-side as DIR/stdout and
DIR/stderr, written live, owned by the host user, and there when run()
returns. The console stays on stdout for diagnostics; the CLI exposes
the flag as --capture.

crates/ecrou-cli/src/cli.rs

@@ -49,6 +49,10 @@ pub struct RunArgs {
     /// Suppress console noise
     #[arg(long)]
     pub quiet: bool,
+    /// Capture the payload's stdout and stderr into files in this
+    /// directory: DIR/stdout and DIR/stderr. Created when missing.
+    #[arg(long)]
+    pub capture: Option<std::path::PathBuf>,
     /// Ignore the cache and download a fresh image
     #[arg(long)]
     pub no_cache: bool,