feat(plugin_builder): default to editable installs

[mofojed]

Switch tools/plugin_builder.py to install plugins with pip install -e
by default so Python source edits take effect without rebuilding wheels.
Add a --wheel flag to opt back into the legacy build-wheel + install flow.

• New run_install_editable() runs pip install -e "<plugin>[all]" per
  plugin, with --force-reinstall --no-deps when --reinstall is passed
  (useful for refreshing JS bundles or re-linking entry points).
• handle_args branches on --wheel: default skips run_build and uses the
  editable installer; --wheel keeps the existing wheel path unchanged.
• --build without --wheel prints a one-line note that it is ignored.
• Bump setuptools requirement to >=64 for PEP 660 editable installs of
  the find_namespace: packages used by every plugin.
• Update the build-plugin skill docs to describe the new default and the
  --wheel opt-out.