provenance: set POC_PROVENANCE for agent subprocesses, Jaccard initial strength

Agent subprocess calls now set POC_PROVENANCE=agent:{name} so any
nodes/links created via tool calls are tagged with the creating agent.
This makes agent transcripts indistinguishable from conscious sessions
in format — important for future model training.

new_relation() now reads POC_PROVENANCE env var directly (raw string,
not enum) since agent names are dynamic.

link-add now computes initial strength from Jaccard similarity instead
of hardcoded 0.8. New links start at a strength reflecting actual
neighborhood overlap.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

poc-memory/src/main.rs

@@ -1642,14 +1642,19 @@ fn cmd_link_add(source: &str, target: &str, reason: &[String]) -> Result<(), Str
         return Ok(());
     }
 
+    // Compute initial strength from Jaccard neighborhood similarity
+    let graph = store.build_graph();
+    let jaccard = graph.jaccard(&source, &target);
+    let strength = (jaccard * 3.0).clamp(0.1, 1.0);
+
     let rel = store::new_relation(
         source_uuid, target_uuid,
-        store::RelationType::Link, 0.8,
+        store::RelationType::Link, strength,
         &source, &target,
     );
     store.add_relation(rel)?;
     store.save()?;
-    println!("Linked: {} → {} ({})", source, target, reason);
+    println!("Linked: {} → {} (strength={:.2}, {})", source, target, strength, reason);
     Ok(())
 }