CLI, agent and absorb; stop tracking target/

Every Go command with the same flags, prompts and messages; the agent is
wire-compatible with Go's; tests drive the real Rust and Go binaries.

Co-Authored-By: Claude <noreply@anthropic.com>

src/main.rs

@@ -1 +1,7 @@
-fn main() {}
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//! vault: a post-quantum encrypted file vault.
+
+fn main() {
+    std::process::exit(vault::cli::main());
+}