feat: implement prompt queue management (CLI-33)

[szmania]

Purpose

This PR implements the prompt queue management feature (CLI-33), allowing users to queue prompts for deferred processing, view the queue, insert prompts at specific positions, and selectively remove items.

Changes

• Core Logic (cecli/commands/core.py):
  • Added prompt_queue (list), _queue_counter (int), _queue_lock (ThreadSafeEvent), and _processing_queue (bool) to the Commands class.
  • Implemented _process_queued_prompts(preproc), which iterates through the queue and executes each item using coder.run_one.
  • Updated Commands.execute to trigger the queue processing in the finally block, ensuring it runs as soon as a command finishes.
  • Added helper methods: _enqueue_prompt, _insert_prompt, _dequeue_prompt, _get_queue_length, and _clear_queue.
• New Commands:
  • /queue <text>: Adds a prompt to the FIFO queue and returns the assigned position.
  • /insert-queue [index] <text>: Inserts a prompt at a specific position in the queue (front by default, or at a given index).
  • /list-queue: Displays all currently queued prompts with their IDs and timestamps.
  • /remove-queue <index|*>: Removes a specific prompt by index or clears the entire queue using a wildcard. Includes an interactive selection mode if no index is provided.
• Registration:
  • Updated cecli/commands/__init__.py to import and register the new commands in the CommandRegistry.

Design Considerations

• Session-bound: The queue is in-memory and session-specific, ensuring it does not persist across restarts.
• Thread Safety: Used asyncio.Lock (via ThreadSafeEvent patterns where appropriate) to ensure safe concurrent access to the queue.
• Recursion Guard: Implemented a guard to ensure management commands (/queue, /insert-queue, /list-queue, /remove-queue) do not accidentally trigger auto-processing recursively.
• Safety Limits: Added a hard limit of 100 items and a character limit (10,000) to prevent memory exhaustion or infinite loops.

Documentation

• Updated cecli/website/docs/usage/commands.md to document the /insert-queue command alongside the other queue management commands.

Testing

• Unit tests for queue logic in core.py.
• Integration tests for command classes.
• E2E tests for full queue lifecycle.
• Regression tests for existing command integrity.