devex: changelog system

[IMB11]

We really suck at changelogs - we should be documenting everything we do - this will encourage it.

• Introduces a fragment-based changelog system using YAML files in .github/changelog/, with scripts to create and bake them into packages/blog/changelog.ts
• Moves changelog data from @modrinth/utils to @modrinth/blog
• Adds CI workflow to remind PR authors to include a changelog fragment, with automatic comment cleanup when one is added - it's always going to be optional though.
• Adds automatic GitHub release creation with changelog body to the Modrinth App release workflow - will document this in Notion.
• new-changelog script now supports creating multiple entries in a single run, with content-hashed filenames

Workflow changes include:

• pnpm scripts new-changelog — interactive prompt to create changelog fragments
• pnpm scripts bake-changelog -- --product <name> [--version X.Y.Z] — consumes fragments and inserts entries into changelog.ts
• pnpm scripts bake-changelog -- --product app --version X.Y.Z --extract — outputs the baked body for a version (used by CI)
• pnpm scripts bake-changelog -- --product web is the same as running it for both hosting + platform.

CI changes:

• changelog-check.yml — on PRs, posts a bot comment if no changelog fragment is present; minimizes it as outdated once one is added
• theseus-release.yml — now extracts the app changelog and creates a GitHub release with it

Also did some small improvements to the changelog.vue page

[modrinth-bot]

Note

No changelog fragment was found for this pull request. If this PR includes user-facing changes, run pnpm scripts new-changelog to create one.

[github-actions]

Frontend previews

Last deployed commit is db501bc76a56553e50da6e2f9c7dfec1e6d8808e

Environment	URL
staging	https://git-db501bc7-frontend-staging.modrinth.workers.dev
production	https://git-db501bc7-frontend.modrinth.workers.dev