cleanup: remove dead migrate module, fix stale comment

migrate.rs was a one-time markdown→capnp conversion that's long done.
Remove it and update the identity.rs comment to reference the new
~/.consciousness/ path.

src/lib.rs

@@ -59,7 +59,7 @@ pub mod memory_capnp {
 pub use hippocampus::{
     store, graph, lookups, cursor, query,
     similarity, spectral, neuro, counters,
-    transcript, migrate, memory,
+    transcript, memory,
 };
 pub use hippocampus::query::engine as search;
 pub use hippocampus::query::parser as query_parser;