provenance: convert from enum to freeform string

The Provenance enum couldn't represent agents defined outside the
source code. Replace it with a Text field in the capnp schema so any
agent can write its own provenance label (e.g. "extractor:write",
"rename:tombstone") without a code change.

Schema: rename old enum fields to provenanceOld, add new Text
provenance fields. Old enum kept for reading legacy records.
Migration: from_capnp_migrate() falls back to old enum when the
new text field is empty.

Also adds `poc-memory tail` command for viewing recent store writes.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

poc-memory/src/agents/digest.rs

@@ -224,7 +224,7 @@ fn generate_digest(
     let digest = call_sonnet("digest", &prompt)?;
 
     let key = digest_node_key(level.name, label);
-    store.upsert_provenance(&key, &digest, store::Provenance::AgentDigest)?;
+    store.upsert_provenance(&key, &digest, "digest:write")?;
     store.save()?;
     println!("  Stored: {}", key);