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/agent/mod.rs

@@ -553,7 +553,7 @@ impl Agent {
     }
 
     pub async fn compact(&self) {
-        match crate::config::reload_for_model(&self.app_config, &self.prompt_file) {
+        match crate::config::reload_for_model(&self.app_config, &self.prompt_file).await {
             Ok(personality) => {
                 let mut ctx = self.context.lock().await;
                 // System section (prompt + tools) set by new(), don't touch it