provenance: add label() method, show provenance in history output

Move provenance_label() from query.rs private function to a pub
label() method on Provenance, eliminating duplication. History command
now shows provenance, human-readable timestamps, and content size for
each version.

Handle pre-migration nodes with bogus timestamps gracefully instead
of panicking.

src/store/types.rs

@@ -247,6 +247,27 @@ pub enum Provenance {
     AgentDecay,
 }
 
+impl Provenance {
+    pub fn label(&self) -> &'static str {
+        match self {
+            Self::Manual                    => "manual",
+            Self::Journal                   => "journal",
+            Self::Agent                     => "agent",
+            Self::Dream                     => "dream",
+            Self::Derived                   => "derived",
+            Self::AgentExperienceMine       => "agent:experience-mine",
+            Self::AgentKnowledgeObservation => "agent:knowledge-observation",
+            Self::AgentKnowledgePattern     => "agent:knowledge-pattern",
+            Self::AgentKnowledgeConnector   => "agent:knowledge-connector",
+            Self::AgentKnowledgeChallenger  => "agent:knowledge-challenger",
+            Self::AgentConsolidate          => "agent:consolidate",
+            Self::AgentDigest               => "agent:digest",
+            Self::AgentFactMine             => "agent:fact-mine",
+            Self::AgentDecay                => "agent:decay",
+        }
+    }
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize, rkyv::Archive, rkyv::Serialize, rkyv::Deserialize)]
 #[archive(check_bytes)]
 pub enum Category {