chore: add snapshot CLI binary and lib re-export

src/bin/snapshot.rs

@@ -0,0 +1,3 @@
+fn main() {
+    antigravity_proxy::snapshot::run_cli();
+}

src/lib.rs

@@ -0,0 +1,3 @@
+//! Antigravity proxy library — shared modules for main proxy and tools.
+
+pub mod snapshot;