feat: add session export cli command

[paralin]

opencode session export [sessionID]

export session transcript to file

Positionals: sessionID session id to export

Options: -o, --output output file path -f, --format output format [string] [choices: "markdown", "json"] [default: "markdown"]

[paralin]

ping @thdxr need this for https://github.com/s4wave/ocpipe which shells out to opencode session export