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>