Remove ContextSource::File — all identity in store now

Identity files migrated to memory nodes:
- identity, core-personality, reflections, where-am-i

Removed:
- ContextSource::File enum variant
- File source parsing and handling
- load_memory_file helper function

Config now only supports Store and Journal sources.

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

src/config.rs

@@ -36,7 +36,6 @@ pub enum ContextSource {
     #[serde(alias = "")]
     #[default]
     Store,
-    File,
     Journal,
 }
 
@@ -320,7 +319,6 @@ impl Config {
                     .unwrap_or_default();
 
                 let source = match obj.get("source").and_then(|v| v.as_str()) {
-                    Some("file") => ContextSource::File,
                     Some("journal") => ContextSource::Journal,
                     _ => ContextSource::Store,
                 };