## Why
Automatic goal continuations can repeatedly return empty final answers without
making progress. Stop this loop by marking the goal as `blocked` after three
consecutive empty turns with no other activity.
## What changed
- Observe completed turn items through a new `on_item_completed` lifecycle hook.
- Track empty final answers only for automatically admitted goal turns, resetting
the streak on activity, user turns, or goal changes.
- Preserve normal turn completion and streamed message deltas when blocking a goal.
## Testing
Add accounting coverage for the three-turn threshold and streak resets, plus
app-server tests for empty continuations and recovery through final-answer text,
commentary, or tool activity.
GitOrigin-RevId: 4b9d2cb2e306b0adc316972429cc35000115b88b