chore: consolidate CEF project structure

What does this merge request do and why?

chore: consolidate CEF project structure

• Remove promptlib code and dependencies.
• Move ELI5 to root and rename to CEF.
• Consolidate dependencies and toolings.

Out of scope

The following work items will be in a separate MR.

• Fix documentation

How to set up and validate locally

1. Check out to this merge request's branch.
2. Run the follow command to confirm the CLI loaded correctly.

   ❯ poetry run cef --help
                                                                                                              
    Usage: cef [OPTIONS] COMMAND [ARGS]...                                                                    
                                                                                                              
   ╭─ Options ───────────────────────────────────────────────────────────────────────────────────────────────╮
   │ --help          Show this message and exit.                                                             │
   ╰─────────────────────────────────────────────────────────────────────────────────────────────────────────╯
   ╭─ Commands ──────────────────────────────────────────────────────────────────────────────────────────────╮
   │ code-suggestions                                                                                        │
   │ duo-chat                                                                                                │
   │ ai-gateway                                                                                              │
   │ duo-code-review                                                                                         │
   │ duo-workflow                                                                                            │
   │ root-cause-analysis                                                                                     │
   │ vulnerability-resolution                                                                                │
   │ datasets                                                                                                │
   ╰─────────────────────────────────────────────────────────────────────────────────────────────────────────╯

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Contributes to #634 (closed)