memory: fix timestamp and provenance on agent writes

Two bugs: upsert_provenance didn't update node.timestamp, so history
showed the original creation date for every version. And native memory
tools (poc-agent dispatch) didn't set POC_PROVENANCE, so all agent
writes showed provenance "manual" instead of "agent:organize" etc.

Fix: set node.timestamp = now_epoch() in upsert_provenance. Thread
provenance through memory::dispatch as Option<&str>, set it via
.env("POC_PROVENANCE") on each subprocess Command. api.rs passes
"agent:{name}" for daemon agent calls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/store/ops.rs

@@ -61,6 +61,7 @@ impl Store {
             let mut node = existing.clone();
             node.content = content.to_string();
             node.provenance = provenance.to_string();
+            node.timestamp = now_epoch();
             node.version += 1;
             self.append_nodes_unlocked(std::slice::from_ref(&node))?;
             self.nodes.insert(key.to_string(), node);