FEAT: Add integrated Tasks/Todo workflow and optional retroactive Product Requirements Document (PRD) generation

[timosaarinen]

Is your feature request related to a problem? Please describe.
Currently, Promptly doesn't have a built-in way to define or track project tasks or requirements in a structured way. This makes it harder to plan features, share requirements with LLMs, or break work down into actionable tasks inside the app. I often end up tracking tasks in a manual TASKS.md file that I provide as context for an LLM - or just copy-pasting tasks from an external document into a prompt.

Describe the solution you'd like
I’d like Promptly to support:

1. A command to create an initial Product Requirements Document (PRD.md) for the project, or to retroactively assemble one by giving the repo as additional context to my prompt. This could use an LLM to generate the first draft based on the project context, or let the user manually edit. Currently, a PRD can be created manually or with AI assistance, but Promptly could help by providing the existing project files as context - especially retroactively.
2. A command to ask the LLM to break the PRD or user-given free-form plan in a prompt into detailed tasks, possibly writing a structured tasks.json file (if TASKS.md is too difficult to make reliably machine-readable without LLM intervention). Each task would have detailed metadata (title, description, status, etc.), with the ability to have per-task detail files if needed.
3. Instructions for the LLM to read and update the task file(s) in each prompt - at least for Architect/ArchitectMax/Edit modes when working with an existing task. This could be optional, perhaps controlled by a checkbox in the UI.
4. A new "Tasks" panel in the UI that lists all tasks from tasks.json, lets me view/edit details, one-click create a prompt to do the task, and possibly update tasks using LLM suggestions. The panel would be grayed out or display an onboarding message if there’s no .promptly/tasks.json file yet.

Describe alternatives you've considered

• Using a plain TASKS.md or TODO.md for tasks, but this is harder to reliably machine-parse for UI viewing.
• Managing tasks in external project management tools (Trello, GitHub Issues, etc.), although integration from Promptly to these could be useful.
• Using JSONL instead of a single JSON file, but a structured JSON file seems easiest for UI integration and future LLM updates.
• Prompt Mode to update the task file(s), but this should be automatically done for each mode and prompt that works with tasks.

Additional context

• See projects like Taskmaster or Ryan Carson’s Cursor rules for good reference implementations of structured task workflows.
• Ideally, the tasks.json format should be easy for both humans and LLMs to update, with room for expansion (labels, priorities, due dates, etc.).