Storybook globs a nonexistent `src/welcome.mdx`

[spaceninja]

.storybook/main.mjs globs src/welcome.mdx, which does not exist. Every Storybook
build and dev-server start prints:

▲ No story files found for the specified pattern: src/welcome.mdx

Why it's there

src/welcome.stories.mdx was added in #535 and deleted in #1220 ("Leverage markdown
docs"), but the stories glob that pointed at it was never updated. It has been a
dangling reference ever since:

// We load the welcome story separately so it will be the first sidebar item.
stories: ['../src/welcome.mdx', '../src/**/*.stories.js', '../src/**/*.mdx'],

#2405 did not introduce this — it only renamed the pattern from welcome.stories.mdx to
welcome.mdx along with every other .stories.mdx file. What changed is that Storybook
10 reports unmatched patterns, so a reference that had been silently dead for years is
now visible on every build.

What to decide

The entry matches nothing, so removing it changes no behaviour — including sidebar
order, which is currently decided entirely by the two globs that follow. So this is
either:

• Delete the glob entry and its comment, accepting that there is no welcome page and
  the first sidebar item is whatever sorts first; or
• Restore an intentional welcome page, if the leading comment still describes
  something we want. Note that the sidebar's first item is currently src/docs/intro.mdx,
  which may already be serving that purpose — in which case the comment is what's stale,
  not the glob.

Small either way, but it wants a decision rather than a silent deletion.

Found while working on the lint & format PR of #2391.

[spaceninja]

Decision: delete the glob entry and its comment.

src/docs/intro.mdx already sorts first and is serving the purpose the comment describes, so the comment is what is stale rather than the glob. No welcome page to restore.

The whole change is one line in .storybook/main.mjs:

// before
// We load the welcome story separately so it will be the first sidebar item.
stories: ['../src/welcome.mdx', '../src/**/*.stories.js', '../src/**/*.mdx'],

// after
stories: ['../src/**/*.stories.js', '../src/**/*.mdx'],

Acceptance: npm run build-storybook no longer prints No story files found for the specified pattern, and the sidebar's first item is unchanged. No changeset — .storybook is not in the published files.

[spaceninja]

Scheduled as part of #2440, a worklist for an agent-driven run. That issue carries the batching and the shared ground rules; the decision for this issue is in the comment above.