Preserve empty objects in MCP JSON config files

[mwikala]

Summary

This fixes plain JSON MCP config updates so existing empty objects are preserved when Boost adds a server entry.

Previously, updating an existing opencode.json that contained an empty object such as "oauth": {} would rewrite it as "oauth": []. OpenCode validates its config schema on startup, so this prevented OpenCode from loading the config after Boost installed its MCP server entry.

The change keeps decoded plain JSON objects as objects while still supporting the existing array-based path used for new config creation.

Benefit to End Users

Users with an existing OpenCode MCP config can install Boost without making their OpenCode configuration invalid. Empty object fields such as oauth remain valid objects, allowing OpenCode to continue loading normally.

Backwards Compatibility

This should not break existing behavior because json_encode supports both arrays and decoded objects. New file creation still uses the existing array-based config path, and existing plain JSON files now retain their original object/array shapes more accurately.

[github-actions]

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