fix(installer): write OpenCode 2 native MCP shape with codemode:false (#1698) (#1768)

OpenCode 2 exposes MCP tools through Code Mode by default; `codemode: false`
only survives on `mcp.servers.<name>` with `disabled`. The installer wrote the
v1 `mcp.codegraph` + `enabled` shape, so the opt-out was dropped on normalize.

Write `mcp.servers.codegraph` with `disabled: false` and `codemode: false`,
migrate a leftover v1 entry on re-install, uninstall either shape, and keep
printConfig / README / AGENTS.md in sync.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

README.md

@@ -854,7 +854,7 @@ is written):
 - **Claude Code**
 - **Cursor**
 - **Codex CLI**
-- **opencode**
+- **opencode** — MCP entry is OpenCode 2's `mcp.servers.codegraph` with `codemode: false` (keeps `codegraph_explore` on the native tool list; `codegraph install` migrates the older `mcp.codegraph` shape)
 - **Hermes Agent**
 - **Gemini CLI**
 - **Antigravity IDE**