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>

src/targets/opencode.ts

@@ -58,12 +58,16 @@ export async function writeOpenCodeBundle(outputRoot: string, bundle: OpenCodeBu
   const openCodePaths = resolveOpenCodePaths(outputRoot)
   await ensureDir(openCodePaths.root)
 
+  const hadExistingConfig = await pathExists(openCodePaths.configPath)
   const backupPath = await backupFile(openCodePaths.configPath)
   if (backupPath) {
     console.log(`Backed up existing config to ${backupPath}`)
   }
   const merged = await mergeOpenCodeConfig(openCodePaths.configPath, bundle.config)
   await writeJson(openCodePaths.configPath, merged)
+  if (hadExistingConfig) {
+    console.log("Merged plugin config into existing opencode.json (user settings preserved)")
+  }
 
   const agentsDir = openCodePaths.agentsDir
   for (const agent of bundle.agents) {