feat: add streamd daily command

`streamd daily [YYYYMMDD]` opens the earliest existing daily file
(file_type=daily) whose timestamp falls within the target date.
If no such file exists it creates `<timestamp>_daily.md` and opens it.
Date defaults to today in the repository timezone.

src/cli/args.rs

@@ -60,6 +60,12 @@ pub enum Commands {
         debug: bool,
     },
 
+    /// Open or create the daily entry for a given date
+    Daily {
+        /// Date in YYYYMMDD format (defaults to today in configured timezone)
+        date: Option<String>,
+    },
+
     /// Generate shell completions
     Completions {
         /// Shell to generate completions for