[FEATURE]: output session_id after a cli run

[phall1]

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

the --continue flag is useful, but not if you're running multiple opencodes concurrently. this feature would be a minimal, addition to output the session id after an opencode run to enable:

1. faster continuation after a run, if running concurrent sessions where the --continue would not work
2. enable better scripting

POSIX scripting patterns:

# Chain sessions (JSON mode — cleanest):
SID=$(opencode run "first task" --format json | jq -r 'select(.type=="session.complete") | .sessionID')
opencode run --session "$SID" "continue"

# Chain sessions (default mode — stderr parsing):
SID=$(opencode run "first task" 2>&1 >/dev/null | grep '^session:' | awk '{print $2}')
opencode run --session "$SID" "continue"

# Simple — just see it in terminal output:
opencode run "do something"
# ... output ...
# session: ses_abc123

Since UI.println writes to stderr, the session ID never pollutes stdout — piping opencode run "msg" | other_tool works cleanly, and the session ID still appears in the terminal.