Docsite changelog should default to core and show an empty state

[nynexman4464]

Problem

The changelog page selects the first package alphabetically, so it opens on @astryxdesign/cli instead of the primary @astryxdesign/core package.

Empty release sections render as bare version headings, which makes the changelog appear incomplete. Theme packages also crowd the primary package selector.

Expected behavior

• Select @astryxdesign/core initially when the URL has no package query.
• Support direct links to visible packages through ?package=..., updating the URL only when the user changes tabs.
• Keep theme packages out of the primary changelog tabs.
• Show a clear "No changes in this release." message beneath empty version headings.