fix: restore CHANGELOG.md symlink and add to .prettierignore

The root CHANGELOG.md is a git symlink (mode 120000) to
./packages/mermaid/CHANGELOG.md. Commit dbfde57 inadvertently added a
trailing newline to the symlink blob via prettier, which on Linux CI
creates a broken symlink (the target path includes a literal newline).

- Restore the symlink blob to the original 31 bytes (no trailing newline)
- Add CHANGELOG.md to .prettierignore so prettier never touches it again

.prettierignore

@@ -1,4 +1,6 @@
 dist
+# Symlink to packages/mermaid/CHANGELOG.md — prettier must not add a trailing newline
+CHANGELOG.md
 cypress/platform/xss3.html
 .cache
 .pnpm-store

CHANGELOG.md

@@ -1 +1 @@
-./packages/mermaid/CHANGELOG.md
+./packages/mermaid/CHANGELOG.md