cli: `opencode [prompt]` start opencode with prefilled user message

[nitsanavni]

Also, support piping the prompt from stdin:

cat my-prompt.md | opencode
echo '/my-custom-slash-command' | opencode 

[rekram1-node]

This got fixed in #1230

you can't do opencode [prompt] but you can do very similar:

cat text.txt | opencode -p "explain file:"