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/commands/mod.rs

@@ -1,4 +1,5 @@
 pub mod completions;
+pub mod daily;
 pub mod edit;
 pub mod new;
 pub mod timesheet;