updater: support per-package changelogs in monorepos #182

Summary

The updater only looks for a changelog at the repository root. Monorepos keep one changelog per package, e.g. getsentry/toolkit has the CLI's in packages/cli/CHANGELOG.md and no root CHANGELOG.md.

What happens

• PR description: get-changelog.ps1 finds no root changelog at either tag, so it falls back to cloning the whole repository and listing the commits between the two tags. For a monorepo these include every package's commits (MCP server, logger, docs, CI), not only the tracked one.
• Changelog entry: update-changelog.ps1 links to <repo>/blob/<main>/CHANGELOG.md#<anchor>, which is a 404.

Suggestion

An optional changelog-path input (e.g. packages/cli/CHANGELOG.md, defaulting to the current root lookup) used for:

• fetching the changelog at both tags in get-changelog.ps1
• the [changelog] link in update-changelog.ps1
• optionally scoping the commit-based fallback to the package directory (git log -- packages/cli)