fix: keep line after deleting empty bullet

[dkarter]

Motivation

Pressing Return on an empty bullet with delete_last_bullet_if_empty enabled removed the whole line, moving the cursor up. The spec now requires the bullet marker to be cleared while keeping the cursor on that line.

Summary of Changes

• Route whitespace-only bullet text through the shared empty-bullet handling.
• Remove the special-case insert-mode deletion path that used ddi.
• Update LC-003 coverage and spec text to assert that the blank line remains.

Testing

• mise run test:spec LC-003
• mise run test:spec LC-004
• mise run test:spec LC-005
• mise run openspec:check
• mise run test
• mise run fmt:check

Dependencies/Special Considerations

none