Add a --prompt flag to the project describe command

[duizendstra]

The project describe command is currently interactive only, which is great for guided use.

However, for scripting and automation (e.g., in CI or when piping from another command), it would be very powerful to provide the prompt text directly via a command-line flag.

Proposed Solution:
Add a --prompt (or -p) flag to the contextvibes project describe command.

• If the flag is used, its value is taken as the prompt.
• If the flag is omitted, the command should fall back to the current interactive behavior.

This would enable workflows like:
contextvibes project describe --prompt "My detailed task description..."

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Context

• CLI Version: dev
• OS/Arch: linux/amd64
• Filed by: @duizendstra