agents: extract run_and_apply, eliminate dead split-plan.md

- Add run_and_apply() — combines run_one_agent + action application
  into one call. Used by daemon job_consolidation_agent and
  consolidate_full, which had identical run+apply loops.

- Port split_plan_prompt() to use split.agent via defs::resolve_placeholders
  instead of loading the separate split-plan.md template. Make
  resolve_placeholders public for this.

- Delete prompts/split-plan.md — superseded by agents/split.agent
  which was already the canonical definition.

poc-memory/src/agents/prompts.rs

@@ -366,12 +366,16 @@ pub fn format_split_plan_node(store: &Store, graph: &Graph, key: &str) -> String
     out
 }
 
-/// Build split-plan prompt for a single node (phase 1)
+/// Build split-plan prompt for a single node (phase 1).
+/// Uses the split.agent template with placeholders resolved for the given key.
 pub fn split_plan_prompt(store: &Store, key: &str) -> Result<String, String> {
+    let def = super::defs::get_def("split")
+        .ok_or_else(|| "no split.agent file".to_string())?;
     let graph = store.build_graph();
-    let topology = format_topology_header(&graph);
-    let node_section = format_split_plan_node(store, &graph, key);
-    load_prompt("split-plan", &[("{{TOPOLOGY}}", &topology), ("{{NODE}}", &node_section)])
+    // Override the query — we have a specific key to split
+    let keys = vec![key.to_string()];
+    let (prompt, _) = super::defs::resolve_placeholders(&def.prompt, store, &graph, &keys, 1);
+    Ok(prompt)
 }
 
 /// Build split-extract prompt for one child (phase 2)