fix(line-editor): Ctrl+K kills to end of line

What

Ctrl+K (0x0B) in the interactive line editor was wired to clearScreen() — duplicating Ctrl+L and leaving the documented emacs kill-to-end-of-line unreachable. Ctrl+K is daily muscle memory for truncating a line, so this is a real papercut.

• Ctrl+K now calls killToEnd() (which also pushes the killed text to the kill ring, so Ctrl+Y can yank it back). Clear-screen stays on Ctrl+L (0x0C).
• Also removes a stray no-op self.killToEnd() self-recursion at the end of killToEnd() (it returned immediately via the cursor >= length guard, but was clearly unintended).

Verification

• zig build clean.
• PTY test: with the cursor before DROP in echo keepDROP, Ctrl+K leaves echo keep and runs it → prints keep.

Two-line behavioral change in src/utils/terminal/line_editor.zig; no API changes.