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/sync/json-config.ts

@@ -18,8 +18,8 @@ export async function mergeJsonConfigAtKey(options: {
   const merged = {
     ...existing,
     [key]: {
-      ...existingEntries,
       ...incoming,
+      ...existingEntries, // existing user entries win on conflict
     },
   }