feat(tasks): implement clear_due_date via FiZ \$empty sentinel (v0.4.16)

The Family Wall API (FiZ framework) uses the special string "\$empty"
to clear optional date fields. Sending dueDate=\$empty to taskupdate2
reliably removes the due date (verified via direct API probing).

- update_task(clear_due_date=True) now sends dueDate=\$empty instead
  of returning an error
- Remove the "not supported" limitation message from the docstring
- Update SPEC.md, README.md, CLAUDE.md to document the discovery
- Bump version to 0.4.16

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -2,7 +2,7 @@
 
 MCP server for [Family Wall](https://www.familywall.com) -- read and manage your family's circles, lists, and tasks directly from Claude.
 
-## Features (v0.4.15)
+## Features (v0.4.16)
 
 ### Read
 
@@ -16,7 +16,7 @@ MCP server for [Family Wall](https://www.familywall.com) -- read and manage your
 ### Write (with confirmation prompt)
 
 - `create_task` -- create a new task in a list (supports `category_id`, `due_date`, `assignee_ids`)
-- `update_task` -- update text, description, category, due date, assignees, or move to a different list (note: clearing a due date is not supported by the Family Wall API)
+- `update_task` -- update text, description, category, due date, assignees, or move to a different list; supports `clear_due_date=True` to remove a due date
 - `toggle_task` -- mark a task complete or reopen it
 - `delete_task` -- permanently delete a task
 - `create_category` -- create a custom category for a shopping list (with optional icon)