Preserve all config fields when updating plugin state

[Copilot]

The saveConfig() calls in enablePlugin and disablePlugin were only writing enabledPlugins and lastUsed, dropping any other fields that might exist in the config.

Changes:

• Spread existing config object before overriding fields in both functions
• Handle null config in enablePlugin with ...(config ?? {})

// Before
await saveConfig({
  enabledPlugins: [...enabledPlugins, pluginName],
  lastUsed: new Date().toISOString(),
});

// After
await saveConfig({
  ...(config ?? {}),
  enabledPlugins: [...enabledPlugins, pluginName],
  lastUsed: new Date().toISOString(),
});

This ensures forward compatibility if additional config fields are added.

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