fix(install): merge config instead of overwriting on opencode target

The sync path's mergeJsonConfigAtKey had incoming entries overwriting
existing user entries on conflict. Reverse the spread order so user
config wins, matching the install path's existing behavior.

Also add merge feedback logging and a test for the sync merge path.

Fixes #125

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/opencode-writer.test.ts

@@ -3,6 +3,7 @@ import { promises as fs } from "fs"
 import path from "path"
 import os from "os"
 import { writeOpenCodeBundle } from "../src/targets/opencode"
+import { mergeJsonConfigAtKey } from "../src/sync/json-config"
 import type { OpenCodeBundle } from "../src/types/opencode"
 
 async function exists(filePath: string): Promise<boolean> {
@@ -254,3 +255,38 @@ describe("writeOpenCodeBundle", () => {
     expect(backupContent).toBe("old content\n")
   })
 })
+
+describe("mergeJsonConfigAtKey", () => {
+  test("preserves existing user entries on conflict", async () => {
+    const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "json-merge-"))
+    const configPath = path.join(tempDir, "opencode.json")
+
+    // User has an existing MCP server config
+    const existingConfig = {
+      model: "my-model",
+      mcp: {
+        "user-server": { type: "local", command: ["uvx", "user-srv"] },
+      },
+    }
+    await fs.writeFile(configPath, JSON.stringify(existingConfig, null, 2))
+
+    // Plugin tries to add its own server and override user-server
+    await mergeJsonConfigAtKey({
+      configPath,
+      key: "mcp",
+      incoming: {
+        "plugin-server": { type: "local", command: ["uvx", "plugin-srv"] },
+        "user-server": { type: "local", command: ["uvx", "plugin-override"] },
+      },
+    })
+
+    const merged = JSON.parse(await fs.readFile(configPath, "utf8"))
+
+    // User's top-level keys preserved
+    expect(merged.model).toBe("my-model")
+    // Plugin server added
+    expect(merged.mcp["plugin-server"]).toBeDefined()
+    // User's server NOT overwritten by plugin
+    expect(merged.mcp["user-server"].command[1]).toBe("user-srv")
+  })
+})