observation: use transcript progress log, mark after success

Wire select_conversation_fragments to use store.is_segment_mined()
instead of scanning _observed-transcripts stub nodes. Segments are
now marked AFTER the agent succeeds (via mark_observation_done),
not before — so failed runs don't lose segments.

Fragment IDs flow through the Resolved.keys → AgentBatch.node_keys
path so run_and_apply_with_log can mark them post-success.

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

poc-memory/src/agents/defs.rs

@@ -226,11 +226,14 @@ fn resolve(
 
         "conversations" => {
             let fragments = super::knowledge::select_conversation_fragments(count);
+            let fragment_ids: Vec<String> = fragments.iter()
+                .map(|(id, _)| id.clone())
+                .collect();
             let text = fragments.iter()
                 .map(|(id, text)| format!("### Session {}\n\n{}", id, text))
                 .collect::<Vec<_>>()
                 .join("\n\n---\n\n");
-            Some(Resolved { text, keys: vec![] })
+            Some(Resolved { text, keys: fragment_ids })
         }
 
         "siblings" | "neighborhood" => {