Add OpenCode as an MCP install target (`wizard mcp add`)

[gewenyu99]

Summary

wizard mcp add installs the PostHog MCP server into the user's AI coding clients. We should add OpenCode (sst/opencode) as a supported target.

Current supported clients

Defined in getSupportedClients() (src/steps/add-mcp-server-to-clients/index.ts): Claude Code, Claude Desktop/Web, Codex, Cursor, VS Code, Zed. OpenCode is a popular terminal AI coding agent with native MCP support and isn't covered.

How OpenCode consumes MCP

OpenCode is config-file based (not a CLI subcommand like claude mcp add / codex mcp add), so it fits the existing DefaultMCPClient file-writer pattern (same shape as Zed / VS Code):

• Config: opencode.json (project) or ~/.config/opencode/opencode.json (global).
• Remote MCP server entry under an mcp key, roughly:

  {
    "mcp": {
      "posthog": {
        "type": "remote",
        "url": "https://mcp.posthog.com/mcp",
        "enabled": true,
        "headers": { "Authorization": "Bearer <key>" }
      }
    }
  }

⚠️ Verify the exact schema (key name mcp, type: "remote" vs "local", enabled, header shape) against the current OpenCode docs before implementing — it has changed over releases.

Implementation sketch

• New src/steps/add-mcp-server-to-clients/clients/opencode.ts extending DefaultMCPClient:
  • getServerPropertyName() → "mcp"
  • getConfigPath() → global OpenCode config path (mac/linux; check Windows)
  • getServerConfig() → remote entry shape above (reuse getNativeHTTPServerConfig, add type/enabled)
  • isClientSupported() → detect OpenCode install / supported platforms
• Register in getSupportedClients() (index.ts).
• Add label/enum + tests mirroring clients/__tests__/claude.test.ts (the DefaultMCPClient reference test).

Context

Filed while hardening the MCP install path (subprocess timeouts + error-tracking the previously-silent config-write failures). Adding OpenCode is follow-up scope, not part of that fix.