Fix page append/prepend overwriting document body

[felix-exon]

Summary

• Make and fetch current document text via
• Merge content client-side and send merged in
• Require explicit text input for append/prepend (, , or )
• Add page command tests for append/prepend merge behavior

Testing

• bun test v1.3.9 (cf6cdbbb)

Fixes #2

[felix-exon]

Applied formatting cleanup (import grouping) in src/commands/page/update.ts. Also ran bun test + typecheck locally.

Commit: d962d33