the frontmatter due should be date... I don't want due anywhere

- Renamed `TaskFrontmatter.due` → `TaskFrontmatter.date`; YAML key on disk is now `date:` instead of `due:`
- Added `#[serde(alias = "due")]` so existing task files with `due:` frontmatter still deserialize correctly
- Updated google_tasks.rs to write `date:` instead of `due:` in generated YAML
- Renamed CLI `--due` flag to `--date`; updated function signature and display string "Due:" → "Date:"

crates/onyx-core/src/google_tasks.rs

@@ -442,7 +442,7 @@ fn render_task_markdown(task: &Task) -> String {
     };
     let mut yaml = format!("id: {}\nstatus: {}\nversion: 1\n", task.id, status_str);
     if let Some(due) = task.date {
-        yaml.push_str(&format!("due: {}\n", due.to_rfc3339()));
+        yaml.push_str(&format!("date: {}\n", due.to_rfc3339()));
     }
     if let Some(parent) = task.parent_id {
         yaml.push_str(&format!("parent: {}\n", parent));

crates/onyx-core/src/storage.rs

@@ -102,7 +102,7 @@ impl From<&Task> for TaskFrontmatter {
         Self {
             id: task.id,
             status: task.status,
-            due: task.date,
+            date: task.date,
             has_time: task.has_time,
             version: task.version,
             parent: task.parent_id,