flatten: move poc-memory contents to workspace root

No more subcrate nesting — src/, agents/, schema/, defaults/, build.rs
all live at the workspace root. poc-daemon remains as the only workspace
member. Crate name (poc-memory) and all imports unchanged.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

build.rs

@@ -0,0 +1,6 @@
+fn main() {
+    capnpc::CompilerCommand::new()
+        .file("schema/memory.capnp")
+        .run()
+        .expect("capnp compile failed");
+}