## Why
Meeting summaries are useful as full notes, but downstream views and file lists also need a very short human-readable blurb without parsing the full markdown body.

## What Changes
- Require the summary agent to provide a one-line summary when calling `write_summary`.
- Reject one-line summaries containing line breaks.
- Store the one-line summary in summary note frontmatter.
- Update default summary-agent instructions to keep the one-line summary very short and concise.

## Impact
- Changes the `write_summary` tool signature.
- Adds summary frontmatter field `oneliner`.
- Adds focused tests for validation, frontmatter output, and instructions.