agents: port knowledge agents to .agent files with visit tracking

The four knowledge agents (observation, extractor, connector,
challenger) were hardcoded in knowledge.rs with their own node
selection logic that bypassed the query pipeline and visit tracking.

Now they're .agent files like the consolidation agents:
- extractor: not-visited:extractor,7d | sort:priority | limit:20
- observation: uses new {{CONVERSATIONS}} placeholder
- connector: type:semantic | not-visited:connector,7d
- challenger: type:semantic | not-visited:challenger,14d

The knowledge loop's run_cycle dispatches through defs::run_agent
instead of calling hardcoded functions, so all agents get visit
tracking automatically. This means the extractor now sees _facts-*
and _mined-transcripts nodes that it was previously blind to.

~200 lines of dead code removed (old runner functions, spectral
clustering for node selection, per-agent LLM dispatch).

New placeholders in defs.rs:
- {{CONVERSATIONS}} — raw transcript fragments for observation agent
- {{TARGETS}} — alias for {{NODES}} (challenger compatibility)

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

poc-memory/src/agents/defs.rs

@@ -160,6 +160,24 @@ fn resolve(
             })
         }
 
+        "conversations" => {
+            let fragments = super::knowledge::select_conversation_fragments(count);
+            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![] })
+        }
+
+        // targets/context: aliases for challenger-style presentation
+        "targets" => {
+            let items = keys_to_replay_items(store, keys, graph);
+            Some(Resolved {
+                text: super::prompts::format_nodes_section(store, &items, graph),
+                keys: vec![],
+            })
+        }
+
         _ => None,
     }
 }