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_cache/src/lib.rs

@@ -16,6 +16,9 @@
 use std::path::{Path, PathBuf};
 
 /// Errors raised by the blob cache.
+// TODO(refactor): Io(String) variant is duplicated across chanora_cache,
+// chanora_storage, and chanora_diagnostics. Could use a shared error type
+// or derive From<std::io::Error> instead of manually wrapping.
 #[derive(Debug, thiserror::Error)]
 pub enum BlobCacheError {
     /// Filesystem I/O error.