feat(mcp): add context7 MCP and disabled_mcps config support

- Add context7 MCP for library documentation lookup
- Add oh-my-opencode.json config file support for per-project settings
- Support disabled_mcps array to selectively disable built-in MCPs
- Update documentation with Configuration section

src/mcp/context7.ts

@@ -0,0 +1,5 @@
+export const context7 = {
+  type: "remote" as const,
+  url: "https://mcp.context7.com/mcp",
+  enabled: true,
+}