Make WYSIWYG editing primary and adopt external file changes

[yeduk3]

What changed

• Make the Edmund-inspired native WYSIWYG editor the default writing surface.
• Keep Markdown source canonical while visually presenting headings, emphasis, links, quotes, code, lists, task checkboxes, and local images.
• Keep Source mode and Preview available as deliberate escape hatches.
• Automatically adopt the on-disk file when an external edit is detected; remove the Reload / Keep Mine conflict prompt.
• Update the README and design notes, and include the existing ship.sh default Markdown-handler improvement.

Why

Switching between raw Markdown and rendered output interrupted writing, and conflict prompts were unnecessary for workflows where an external formatter or sync service owns the file. The new behavior keeps editing native and document-like while making the disk version authoritative.

Validation

• ./build.sh Debug
• git diff --check
• Verified the Debug app on macOS, including WYSIWYG/Preview switching and native task checkboxes.