refactor(core,storage): deduplicate code patterns (TODO-014)

Extract PttCapability event construction helper in events.rs.
Extract ensure_dir() helper in chanora_storage. Add TODO(refactor)
annotations for patterns requiring shared crate or API changes.

crates/chanora_prefetch/src/lib.rs

@@ -194,6 +194,9 @@ impl ServerPrefetcher {
     }
 }
 
+// TODO(refactor): `normalize_host` duplicates `chanora_resolver::normalize_args`
+// host trimming. Both do `host.trim().to_lowercase()`. Could move to a shared
+// utility in chanora_protocol or a tiny chanora_common crate if more crates need it.
 fn normalize_host(host: &str) -> String {
     host.trim().to_lowercase()
 }