## 1. Specification
- [x] Add summary one-liner requirements and scenarios.

## 2. Behavior
- [x] Require `oneliner` in the `write_summary` tool signature.
- [x] Validate `oneliner` does not contain line breaks.
- [x] Write `oneliner` to summary note frontmatter.
- [x] Instruct the summary agent to keep the one-liner very short.

## 3. Verification
- [x] Add focused behavior tests.
- [x] Run relevant tests.
- [x] Validate OpenSpec change.