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- Delete prompts/split-plan.md — superseded by agents/split.agent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/defs.rs

@@ -184,7 +184,7 @@ fn resolve(
 
 /// Resolve all {{placeholder}} patterns in a prompt template.
 /// Returns the resolved text and all node keys collected from placeholders.
-fn resolve_placeholders(
+pub fn resolve_placeholders(
     template: &str,
     store: &Store,
     graph: &Graph,