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>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "mcp-familywall"
-version = "0.4.15"
+version = "0.4.16"
 description = "MCP server for Family Wall — read your family's lists and tasks via Claude"
 readme = "README.md"
 requires-python = ">=3.12"