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/cli/agent.rs

@@ -46,7 +46,7 @@ pub async fn cmd_run_agent(agent: &str, count: usize, target: &[String], query:
             let mut store = store::Store::load()?;
             if let Err(e) = crate::agent::oneshot::run_one_agent(
                 &mut store, agent, count, Some(&[key.clone()]),
-            ) {
+            ).await {
                 println!("[{}] ERROR on {}: {}", agent, key, e);
             }
         }
@@ -55,7 +55,7 @@ pub async fn cmd_run_agent(agent: &str, count: usize, target: &[String], query:
         let mut store = store::Store::load()?;
         crate::agent::oneshot::run_one_agent(
             &mut store, agent, count, None,
-        )?;
+        ).await?;
     }
     Ok(())
 }