Improve release notes

## Problem

Our release notes are mostly generated from PR titles and edited manually before release. This process can be time-consuming as PR titles are not particularly descriptive and user-friendly, so often we need to browse PRs for additional context or rely on LLMs to rewrite the notes for clarity.

## Possible Solution

Enforce adding release notes for every PR before merge. There are tools that help automate this such as [towncrier](https://github.com/twisted/towncrier), which can be used to collect notes from a given directory and rewrite them in a custom/standard format such as [Keep a Changelog](https://github.com/twisted/towncrier/blob/f42c86724a1942768f8f622768b25b157926f73b/docs/markdown.rst).

We can reference the BeeWare project's implementation: https://beeware.org/contributing/guide/how/change-note/

No repository files or tests are named. Start by mapping how pull request titles currently become release notes, then compare the proposed towncrier and BeeWare approaches. Done means a decided workflow that requires release notes before merge and produces clear, consistently formatted release notes.

索引モデルが issue の本文から書いたものです。