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/subconscious/prompts.rs

@@ -212,8 +212,8 @@ pub fn format_health_section(store: &Store, graph: &Graph) -> String {
 }
 
 /// Generate a specific agent prompt with filled-in data.
-pub fn agent_prompt(store: &Store, agent: &str, count: usize) -> Result<AgentBatch, String> {
+pub async fn agent_prompt(store: &Store, agent: &str, count: usize) -> Result<AgentBatch, String> {
     let def = super::defs::get_def(agent)
         .ok_or_else(|| format!("Unknown agent: {}", agent))?;
-    super::defs::run_agent(store, &def, count, &Default::default())
+    super::defs::run_agent(store, &def, count, &Default::default()).await
 }