Redesign pgtui icon as a faceted prompt chevron

Replace the chevron+cursor glyph with a shell-prompt ">" built from two
solid arm triangles meeting at the tip and an equal-sized hollow notch
triangle between them, on one shared green gradient — matching the
faceted, hollow-negative-space language of the pgparser icon. Drop the
unused stale pgtui.png raster.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tianzhou