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/consolidate.rs

@@ -178,7 +178,7 @@ pub fn consolidate_full_with_progress(
 
     // Store the log as a node
     store.upsert_provenance(&log_key, &log_buf,
-        store::Provenance::AgentConsolidate).ok();
+        "consolidate:write").ok();
     store.save()?;
 
     Ok(())