Document todo subtask API usage in Basecamp skill

skills/basecamp/SKILL.md

@@ -431,6 +431,71 @@ basecamp todos sweep --overdue --complete --comment "Done" --in <project>
 
 **Flags:** `--assignee` (todos only - not available on cards/messages), `--status` (completed/incomplete/archived/trashed), `--overdue`, `--list`, `--due`, `--limit`, `--all`
 
+**Todo Subtasks (checklist steps):** Basecamp to-do subtasks are stored as
+`Kanban::Step` records, even when their parent is a normal `Todo`. The regular
+`todos show` response may not include them; use
+`basecamp recordings list --type Kanban::Step` and filter by `parent.id` to
+list/check subtasks for a todo.
+
+```bash
+# Create a subtask under a todo. Use the todo ID in this card-style path.
+basecamp api post /buckets/<project>/card_tables/cards/<parent_todo_id>/steps.json \
+  --data '{"title":"Subtask title"}' \
+  --json
+
+# Read or edit a subtask
+basecamp api get /buckets/<project>/card_tables/steps/<step_id>.json --json
+basecamp api put /buckets/<project>/card_tables/steps/<step_id>.json \
+  --data '{"title":"Updated subtask title"}' \
+  --json
+
+# List subtasks for a todo
+PARENT_TODO_ID=<parent_todo_id> \
+basecamp recordings list --in <project> --type Kanban::Step --all --json \
+  --jq '.data[] | select(.parent.id==(env.PARENT_TODO_ID | tonumber)) | {id,title,status,parent:.parent.id,url}'
+
+# Assign or set a due date. Include the current title when updating metadata.
+basecamp api put /buckets/<project>/card_tables/steps/<step_id>.json \
+  --data '{"title":"Current subtask title","assignee_ids":[<person_id>],"due_on":"<YYYY-MM-DD>"}' \
+  --json
+
+# Complete or reopen a subtask
+basecamp api put /buckets/<project>/card_tables/steps/<step_id>/completions.json \
+  --data '{"completion":"on"}' \
+  --json
+basecamp api put /buckets/<project>/card_tables/steps/<step_id>/completions.json \
+  --data '{"completion":"off"}' \
+  --json
+
+# Trash a subtask from the todo UI by trashing the step record (Kanban::Step)
+basecamp recordings trash <step_id> --in <project> --json
+```
+
+Key points: replace numeric placeholders such as `<parent_todo_id>` and
+`<person_id>` before running the examples. For creating todo subtasks, Basecamp
+accepts the parent todo ID in the `/card_tables/cards/<parent_todo_id>/steps.json`
+path. To list subtasks under a todo, use
+`basecamp recordings list --type Kanban::Step` with the `parent.id` filter shown
+above.
+
+Completed subtasks have `completed: true` and a `completion` object with
+`created_at` and `creator`. Open subtasks have `completed: false` and no
+`completion` object. Trashed subtasks may still be readable directly with
+`status: "trashed"` and `inherits_status: false`, but they no longer appear in
+the todo UI.
+
+In testing with todo-backed steps, these direct `GET` requests returned
+`not_found`: `/card_tables/cards/<parent_todo_id>/steps.json`,
+`/card_tables/cards/<parent_todo_id>.json`, and
+`/todos/<parent_todo_id>/steps.json`. To inspect trashed subtasks, add
+`--status trashed`; archived parents may require `--status archived`.
+
+When updating a todo subtask with the raw API, include the existing `title` along
+with metadata changes; omitting it may reset the step title to `Untitled`. The
+generic `basecamp assign <step_id> --step ...` command is intended for card
+steps and may fail with `Bad Request` for todo-backed steps, so prefer
+`assignee_ids` on the raw step update endpoint for todo subtasks.
+
 ### Todolists
 
 Todolists are containers for todos. Create a todolist before adding todos.