fix: deleteToLineStart joins with previous line when cursor at col 0

[fredsh2k]

Summary

• Fixes deleteToLineStart() doing nothing when the cursor is at column 0 with row > 0
• When cursor is at col 0, calls editBuffer.deleteCharBackward() to remove the newline and join with the previous line

Details

EditBufferRenderable.deleteToLineStart() only deletes when cursor.col > 0. After clearing a line's content, the cursor sits at col 0 and subsequent calls are no-ops. This makes repeated Ctrl+U presses get stuck instead of continuing to delete line-by-line upward.

The fix adds an else if (cursor.row > 0) branch that calls editBuffer.deleteCharBackward() to remove the newline character, matching the behavior users expect from terminals and editors.

Tests

Added 4 new tests in Textarea.editing.test.ts:

• Delete to line start from middle of line (existing behavior)
• Join with previous line when at col 0
• No-op when at row 0 col 0
• Repeated deleteToLineStart across multiple lines

Closes #937