[FEATURE]: Open session selector with -s without a session ID

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

Currently, opencode -s <session-id> resumes a specific session

I'd like to contribute opencode -s which starts the TUI and immediately opens the /session selector.

Update for V2

• v1 uses yargs which made no values default to "": https://github.com/anomalyco/opencode/pull/48719).
• v2 uses effect, which rejects a string flag without a value: https://github.com/anomalyco/opencode/pull/50052

Extra; what I use today (requires fzf jq):

opencode2 session list --format json |
    jq -r '.[] | [.id, (.title // "Untitled"), ((.updated / 1000) | strftime("%Y-%m-%d %H:%M"))] | @tsv' |
    fzf --delimiter=$'\t' --border --height=60% --header=$'Session ID\tTitle\tUpdated' |
    cut -f1 |
    xargs -I{} sh -c 'opencode2 -s "$1" < /dev/tty' sh {}