2.0.13: Plugin scaffold: update-safe user config pattern (standard #7)

ADR plugin-manager_extension-update-flow #7 / RISK-OPS-plugin-config-file-overwrite:
1-click update overwrites plugin files but preserves admin_config, so plugins
must keep site-owner-editable config in DB, not in the overwritten config.php.

- Format/plugin/config.php: documented as DEFAULTS only + example settings block.
- Format/plugin/function.php: ship commented *_effective_config()/*_save_config()
  helpers implementing default(file) + override(DB), using the admin_config
  `<key>_config` slot that AppConfig::uninstall already cleans.
- MakePlugin: template config.php + function.php (were copied verbatim) so the
  Extension_Key placeholder resolves to the real plugin key.
- readme{,_vi}: document the standard, referencing the MFA plugin.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>