OpenCode TUI reads input.subagent_type to display task type. When
subagent_type was missing (e.g., category-only or session continuation),
TUI showed 'Unknown Task'.
Fix:
- category provided: always set subagent_type to 'sisyphus-junior'
(previously only when subagent_type was absent)
- session_id continuation: resolve agent from session's first message
- fallback to 'continue' if session has no agent info
YeonGyu-Kim