Add ps.no_spines() for imshow/pcolormesh/2D-histogram plots

Image- and bin-indexed plots have no meaningful x baseline, so the
themed bottom spine kept by use() doesn't apply to them. Adds an
explicit opt-in to disable all spines on such Axes, documents it in
plotstyle/CLAUDE.md, and updates the showcase notebook's colormap and
multi-panel examples to use it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plotstyle/__init__.py

@@ -13,7 +13,7 @@ scientific plots across presentations and thesis figures.
 from . import colors
 from .annotations import panel_label, style_legend
 from .colors import get_color
-from .figures import FIGSIZES, colorbar, new_figure, savefig
+from .figures import FIGSIZES, colorbar, new_figure, no_spines, savefig
 from .style import reset, use
 
 __all__ = [
@@ -23,6 +23,7 @@ __all__ = [
     "reset",
     "new_figure",
     "colorbar",
+    "no_spines",
     "savefig",
     "FIGSIZES",
     "style_legend",