[FEATURE]: `opencode session export` to markdown or json

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

We need a way to export opencode sessions to Markdown or Json format with a TUI flag.

 % opencode session export -h
opencode session export [sessionID]

export session transcript to file

Positionals:
  sessionID  session id to export                                                           [string]

Options:
  -h, --help        show help                                                              [boolean]
  -v, --version     show version number                                                    [boolean]
      --print-logs  print logs to stderr                                                   [boolean]
      --log-level   log level                   [string] [choices: "DEBUG", "INFO", "WARN", "ERROR"]
      --dir         directory to run in                                                     [string]
  -o, --output      output file path                                                        [string]
  -f, --format      output format       [string] [choices: "markdown", "json"] [default: "markdown"]