docs(chore[spa]): drop spa-copybutton-reinit.js shim; gp-sphinx handles it now

The shim was a project-local workaround for two gp-sphinx bugs in
``spa-nav.js::addCopyButtons``: clone-based template failed on pages
without code blocks, and only ``div.highlight pre`` was iterated
(not the full configured ``copybutton_selector``, so prompt blocks
went buttonless after SPA swap).

Both are fixed upstream in gp-sphinx (#20): inline HTML template and
``window.GP_SPHINX_COPYBUTTON_SELECTOR`` bridge. The shim's
responsibilities collapse into the theme's own code, so remove it
from conf.py's ``app.add_js_file`` list and delete the file.

``prompt-copy.js`` stays — it's unrelated: it implements Markdown-
preserving copy (backtick-wrapping ``<code>`` children) for prompt
admonitions, which sphinx-copybutton's ``innerText``-based copy
flattens. That behavior has nothing to do with SPA navigation and
uses document-level event delegation to survive DOM swaps.

Author: tony

docs/_static/js/spa-copybutton-reinit.js

@@ -142,7 +142,6 @@ def setup(app: Sphinx) -> None:
     _gp_setup(app)
     app.connect("autodoc-process-docstring", _convert_md_xrefs)
     app.add_js_file("js/prompt-copy.js", loading_method="defer")
-    app.add_js_file("js/spa-copybutton-reinit.js", loading_method="defer")
     app.add_css_file("css/project-admonitions.css")