Automate changelog generation from merged PR titles and descriptions

[MariusStorhaug]

Context

Issue #216 captured research ideas from intuit/auto, including automated changelog generation from PR metadata. That research issue is intentionally non-actionable.

Request

Implement a concrete changelog generation step that builds release notes from merged pull request titles and descriptions.

Desired behavior

• Collect merged PRs since the last release tag
• Parse title format and PR description sections
• Group entries by change type labels (Major, Minor, Patch, NoRelease)
• Exclude NoRelease entries by default from user-facing release notes
• Produce markdown changelog output usable in GitHub Releases

Acceptance criteria

• Changelog is generated automatically from merged PRs
• Output includes PR number links and user-facing summaries
• Label/type grouping is deterministic and documented
• NoRelease PRs are excluded from release notes unless explicitly enabled
• A dry-run mode is available for validation in CI

Implementation plan

• Define release window selection logic (from last tag to current)
• Define parsing rules for PR title/body and fallback behavior
• Implement changelog generation step in release workflow
• Add tests/fixtures for mixed PR inputs and edge cases
• Document changelog conventions for contributors