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/knowledge.rs

@@ -138,6 +138,16 @@ fn run_one_agent_inner(
         else { format!("{} tools", def.tools.len()) };
     log(&format!("prompt {}KB, model={}, {}, {} nodes",
         prompt_kb, def.model, tools_desc, agent_batch.node_keys.len()));
+
+    // Guard: reject prompts that would exceed model context.
+    // Rough estimate: 1 token ≈ 4 bytes. Reserve 16K tokens for output.
+    let max_prompt_bytes = 800_000; // ~200K tokens, leaves room for output
+    if agent_batch.prompt.len() > max_prompt_bytes {
+        return Err(format!(
+            "prompt too large: {}KB (max {}KB) — seed nodes may be oversized",
+            prompt_kb, max_prompt_bytes / 1024,
+        ));
+    }
     for key in &agent_batch.node_keys {
         log(&format!("  node: {}", key));
     }