Add task activity summaries

[bborn]

• Generate a concise 2–4 bullet activity summary from task logs on completion and store it in tasks.summary.\n- Render the summary in the task detail view for quick context switching.\n- Add DB helper to update summaries with last_distilled_at.\n\nTests:\n- make test (failed: Go version mismatch go1.24.4 vs go1.25.5)