agents: fix resolve_placeholders infinite loop

The placeholder resolver re-scanned from the beginning of the string
after each expansion. If expanded node content contained {{...}}
patterns (which core-personality does), those got expanded recursively.
Cyclic node references caused infinite string growth.

Fix: track a position offset that advances past each substitution,
so expanded content is never re-scanned for placeholders.

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

poc-memory/src/agents/defs.rs

@@ -611,19 +611,25 @@ pub fn resolve_placeholders(
 ) -> (String, Vec<String>) {
     let mut result = template.to_string();
     let mut extra_keys = Vec::new();
+    let mut pos = 0;
     loop {
-        let Some(start) = result.find("{{") else { break };
-        let Some(end) = result[start + 2..].find("}}") else { break };
-        let end = start + 2 + end;
+        let Some(rel_start) = result[pos..].find("{{") else { break };
+        let start = pos + rel_start;
+        let Some(rel_end) = result[start + 2..].find("}}") else { break };
+        let end = start + 2 + rel_end;
         let name = result[start + 2..end].trim().to_lowercase();
         match resolve(&name, store, graph, keys, count) {
             Some(resolved) => {
+                let len = resolved.text.len();
                 extra_keys.extend(resolved.keys);
                 result.replace_range(start..end + 2, &resolved.text);
+                pos = start + len;
             }
             None => {
                 let msg = format!("(unknown: {})", name);
+                let len = msg.len();
                 result.replace_range(start..end + 2, &msg);
+                pos = start + len;
             }
         }
     }