style(copy): remove em dashes from the site's prose

The em dash is one of the clearest tells of machine-written text, which
is the exact impression this work exists to remove. Rewritten rather
than substituted: where a dash was carrying a real aside the sentence is
split or recast, not patched with a comma.

Covers the About page, the two Callout labels an editor sees in the
admin panel, and PUBLISHING.md, which defines the house style and should
follow it. The rule is now recorded in that house style and in the
spec's voice rules, so it survives this branch.

Code comments are left alone: they are not copy, and the surrounding
codebase uses the same punctuation throughout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/blocks/Callout.ts

@@ -16,8 +16,8 @@ export const Callout: Block = {
       type: 'select',
       defaultValue: 'caveat',
       options: [
-        { label: 'Caveat — what this does not show', value: 'caveat' },
-        { label: 'Note — useful aside', value: 'note' },
+        { label: 'Caveat: what this does not show', value: 'caveat' },
+        { label: 'Note: useful aside', value: 'note' },
       ],
     },
     { name: 'body', type: 'textarea', required: true },