## Why
Short continuation prompts such as `continue` do not contain enough context for
skill selection on their own.
## What changed
- Add the `task_context_fusion_v1` shadow selector, combining the current request
with up to two prior substantive requests and recently relevant skills.
- Record explicit skill intent and successful skill invocations for future turns,
while excluding same-turn observations from predictions.
- Bound retained requests, augmented queries, and skill history, including safe
truncation at UTF-8 character boundaries.
## Testing
Add unit and extension tests for continuation prompts, explicit intent, turn
isolation, cold thread state, bounded history, and unchanged control selectors.
GitOrigin-RevId: 72eca3f0d64620a0d24e95d5675f126ac982f8c7
21cfd369ef