config: move user_name/assistant_name to AppConfig (top level)

These are identity settings, not memory-graph settings. Sat inside the
\`memory\` section only because that's where Config started life. Move
to AppConfig alongside the other top-level stuff.

Readers now pull from \`config::app()\` instead of \`config::get()\`.
subconscious/defs.rs's conversation-building pass still needs Config
for surface_conversation_bytes, so both guards coexist there —
AppConfig's guard is dropped before the per-step await loop so we
don't stall the config-watcher's writer.

show_config picks up the two new fields at the top of its output.
Kent's config already has them hoisted to the top level.

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

src/mind/subconscious.rs

@@ -92,7 +92,8 @@ impl State {
     /// Generate the DMN prompt for the current state, informed by
     /// user presence and error patterns.
     pub fn prompt(&self, ctx: &DmnContext) -> String {
-        let user = &crate::config::get().user_name;
+        let app = crate::config::app();
+        let user = &app.user_name;
 
         let idle_info = if ctx.user_idle < Duration::from_secs(60) {
             format!("{} is here (active recently).", user)