tier-check: support monorepos with multiple SDK packages

[pietrozullo]

We maintain a monorepo (mcp-use/mcp-use) that contains both a Python SDK and a TypeScript SDK. The tier-check command has a couple of issues with this setup:

1. Stable Release detection picks up the wrong release. Our TypeScript canary releases (e.g. mcp-use@1.18.0-canary.3) get detected as the "latest version" instead of the stable Python release (v1.5.2). The check then fails because it sees a pre-release identifier.

2. Spec tracking gap calculation may be picking up the wrong release tag for the same reason, it doesn't know which package's releases to look at.

The release detection logic fetches the latest 20 releases and picks the first one, which doesn't account for repos publishing releases for multiple packages with different tag patterns.

Would it be possible to support a --package or --tag-prefix flag to filter releases by package name (e.g. --tag-prefix python-v or --package mcp-use)?

Also, is tier-check intended as a self-assessment helper for SDK maintainers, or will it be used systematically to determine the official tier of each SDK as described in the SDK Tiering System? Asking because the answer affects how much we need to adapt our release tagging vs. how much the tool should adapt to different repo structures.