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/hippocampus/mod.rs

@@ -15,5 +15,4 @@ pub mod similarity;
 pub mod spectral;
 pub mod neuro;
 pub mod counters;
-pub mod migrate;
 pub mod transcript;