defs.rs: async conversion, remove block_in_place

Convert resolve(), resolve_placeholders(), run_agent() to async.
Use memory_render/memory_query directly with .await instead of
block_in_place wrappers.

Propagate async to callers:
- config.rs: resolve(), load_session(), reload_for_model()
- identity.rs: load_memory_files(), assemble_context_message()
- oneshot.rs: run_one_agent()
- prompts.rs: agent_prompt()

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/unconscious.rs

@@ -265,7 +265,7 @@ pub async fn prepare_spawn(name: &str, mut auto: AutoAgent) -> Result<SpawnResul
     let exclude: std::collections::HashSet<String> = std::collections::HashSet::new();
     let batch = match defs::run_agent(
         &store, &def, def.count.unwrap_or(5), &exclude,
-    ) {
+    ).await {
         Ok(b) => b,
         Err(e) => {
             dbglog!("[unconscious] {} query failed: {}", name, e);