zg install --target opencode writes opencode.json even when the active config is opencode.jsonc #103

Environment

• zg 0.2.1, Linux, Node 24
• OpenCode v2 with the config at ~/.config/opencode/opencode.jsonc (JSONC, with comments)
• OPENCODE_CONFIG not set

What happened
zg install --target opencode wrote a managed zvec_grep MCP entry into ~/.config/opencode/opencode.json. OpenCode reads our opencode.jsonc, so the installer created a shadow file that the agent never loads -- the integration silently does nothing until the entry is added to the .jsonc manually.

Repro

1. Have ~/.config/opencode/opencode.jsonc as your active OpenCode config (no opencode.json present).
2. Run zg install --target opencode --yes.
3. ~/.config/opencode/opencode.json is created with the managed entry; the agent still has no zvec-grep tool.

Expected
The installer should detect an existing opencode.jsonc (or an OPENCODE_CONFIG pointing at one) and write the managed entry there, preserving JSONC comments and unrelated settings the way the Qwen/Qoder paths already do. If both exist, prefer the one OpenCode actually loads and say so in the output.

Workaround we use
Hand-adding the flat entry to opencode.jsonc: "zvec_grep": { "type": "local"|"remote", ... , "enabled": true }. Everything else about the integration (agent toolset, guidance, server) works great once the entry lands in the real config -- thanks for a genuinely useful tool.

Suggested fix
In the opencode target resolution, stat opencode.jsonc before defaulting to opencode.json, and route managed-block writes through a JSONC-preserving editor (the Qwen settings.json path already preserves comments, so the machinery seems to exist).