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/store/ops.rs

@@ -37,13 +37,15 @@ impl Store {
     /// Provenance is determined by the POC_PROVENANCE env var if set,
     /// otherwise defaults to Manual.
     pub fn upsert(&mut self, key: &str, content: &str) -> Result<&'static str, String> {
-        let prov = Provenance::from_env().unwrap_or(Provenance::Manual);
-        self.upsert_provenance(key, content, prov)
+        let prov = Provenance::from_env()
+            .map(|p| p.label().to_string())
+            .unwrap_or_else(|| "manual".to_string());
+        self.upsert_provenance(key, content, &prov)
     }
 
     /// Upsert with explicit provenance (for agent-created nodes).
     /// Holds StoreLock across refresh + check + write to prevent duplicate UUIDs.
-    pub fn upsert_provenance(&mut self, key: &str, content: &str, provenance: Provenance) -> Result<&'static str, String> {
+    pub fn upsert_provenance(&mut self, key: &str, content: &str, provenance: &str) -> Result<&'static str, String> {
         let _lock = StoreLock::acquire()?;
         self.refresh_nodes()?;
 
@@ -53,14 +55,14 @@ impl Store {
             }
             let mut node = existing.clone();
             node.content = content.to_string();
-            node.provenance = provenance;
+            node.provenance = provenance.to_string();
             node.version += 1;
             self.append_nodes_unlocked(std::slice::from_ref(&node))?;
             self.nodes.insert(key.to_string(), node);
             Ok("updated")
         } else {
             let mut node = new_node(key, content);
-            node.provenance = provenance;
+            node.provenance = provenance.to_string();
             self.append_nodes_unlocked(std::slice::from_ref(&node))?;
             self.uuid_to_key.insert(node.uuid, node.key.clone());
             self.nodes.insert(key.to_string(), node);