agents: always use API backend, remove tools field from .agent files

- Remove is_split special case in daemon — split now goes through
  job_consolidation_agent like all other agents
- call_for_def uses API whenever api_base_url is configured, regardless
  of tools field (was requiring non-empty tools to use API)
- Remove "tools" field from all .agent files — memory tools are always
  provided by the API layer, not configured per-agent
- Add prompt size guard: reject prompts over 800KB (~200K tokens) with
  clear error instead of hitting the model's context limit

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

poc-memory/src/agents/llm.rs

@@ -188,7 +188,7 @@ pub(crate) fn call_haiku(agent: &str, prompt: &str) -> Result<String, String> {
 /// otherwise falls back to claude CLI subprocess.
 pub(crate) fn call_for_def(def: &super::defs::AgentDef, prompt: &str) -> Result<String, String> {
     let config = crate::config::get();
-    if config.api_base_url.is_some() && !def.tools.is_empty() {
+    if config.api_base_url.is_some() {
         super::daemon::log_verbose(&def.agent, "llm-backend",
             &format!("API: {}", config.api_base_url.as_deref().unwrap_or("?")));
         let log = |msg: &str| eprintln!("[{}] {}", def.agent, msg);