feat(todo): add numbered tasks, edit, done, and future filtering

Implement smoother todo editing with the following features:
- Display numbered tasks [1], [2], etc. in `streamd todo`
- Add `streamd todo N edit` to open editor at task line
- Add `streamd todo N done` to insert @Done after @Task
- Add `--show-future` flag to include future tasks (hidden by default)

src/cli/args.rs

@@ -13,13 +13,34 @@ pub struct Cli {
     pub command: Option<Commands>,
 }
 
+#[derive(Subcommand)]
+pub enum TodoAction {
+    /// Edit a task by its number
+    Edit {
+        /// Task number to edit
+        number: usize,
+    },
+    /// Mark a task as done
+    Done {
+        /// Task number to mark as done
+        number: usize,
+    },
+}
+
 #[derive(Subcommand)]
 pub enum Commands {
     /// Create a new stream file
     New,
 
     /// Display open tasks
-    Todo,
+    Todo {
+        /// Show tasks with dates in the future
+        #[arg(long)]
+        show_future: bool,
+
+        #[command(subcommand)]
+        action: Option<TodoAction>,
+    },
 
     /// Edit a stream file by position
     Edit {