add --name TEXT option to 'llm prompt' command to set conversation_name

Very often with bigger/longer prompts (or if using prompt for data), conversation name is meaningless, leaving user hard time searching db.

Very often setting name of the conversation by the user at prompt time would solve the problem.

Easy solution:
--name TEXT -> sets conversation_name to TEXT

Better:
--name TEXT -> sets conversation name to TEXT + prompt or system_prompt (meaning this is prefix for name)

Best:
allow --name TEXT be used in 'llm prompt' but also in 'llm logs list --name TEXT' returning all conversations having TEXT in conversation_name

p.s. this would be ultra helpful for use cases like this: https://til.simonwillison.net/llms/claude-hacker-news-themes where I would like to put HN url at the begining of conversation_name :)

Start by reading the implementations of the llm prompt and llm logs list commands, then trace how conversation_name is stored and searched. Resolve which of the proposed --name behaviors is intended; done means the selected behavior works for the command and conversation names can be found as specified.