surface: tag recent nodes as (new) instead of hiding them

Links to nodes created after the conversation window start are
tagged with (new) in memory_render output. The surface prompt
tells the agent not to surface these — they're its own recent
output, not prior memories. Observe can still see and update them.

POC_MEMORIES_OLDER_THAN env var set from the oldest message
timestamp in the conversation window.

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

src/agent/tools/memory.rs

@@ -107,8 +107,9 @@ pub fn dispatch(name: &str, args: &serde_json::Value, provenance: Option<&str>)
             let node = MemoryNode::load(key)
                 .ok_or_else(|| anyhow::anyhow!("node not found: {}", key))?;
             let mut out = format!("Neighbors of '{}':\n", key);
-            for (target, strength) in &node.links {
-                out.push_str(&format!("  ({:.2}) {}\n", strength, target));
+            for (target, strength, is_new) in &node.links {
+                let tag = if *is_new { " (new)" } else { "" };
+                out.push_str(&format!("  ({:.2}) {}{}\n", strength, target, tag));
             }
             Ok(out)
         }