ci: generate human-readable stable release notes from PRs #4653

User Story

As an OpenShell release maintainer, I need stable release notes that summarize merged pull requests in plain language so users can quickly understand what changed and what might affect them.

Description

The stable release workflow currently uses automatically generated notes. Have it publish a concise release body with Quick install, Breaking changes, Bug fixes, Features, and Other sections. Use an explicit Release Notes section from a PR when present, group related PRs into one item, and link every relevant PR. Scope the review with the comparison between the previous stable tag and the new tag.

Context

The release notes should be generated during the stable release workflow and attached to the GitHub Release. Pre-release publishing should keep its current behavior.

Definition of Done

• Stable releases publish short, human-readable notes in the requested sections.
• Notes use explicit PR Release Notes text when supplied and link grouped PRs.
• The release workflow passes the stable tag comparison to the notes generator.
• PR guidance documents the optional Release Notes section.