replace HOME env var panics with dirs::home_dir()

Four expect("HOME not set") calls in config.rs and one unwrap()
in admin.rs would panic if HOME wasn't set. Use dirs::home_dir()
consistently for portability.

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

src/cli/admin.rs

@@ -46,7 +46,7 @@ pub fn cmd_init() -> Result<(), String> {
     let config_path = std::env::var("POC_MEMORY_CONFIG")
         .map(std::path::PathBuf::from)
         .unwrap_or_else(|_| {
-            std::path::PathBuf::from(std::env::var("HOME").unwrap())
+            dirs::home_dir().unwrap_or_default()
                 .join(".consciousness/config.jsonl")
         });
     if !config_path.exists() {