Fix: preserve ALL existing non-.opencode files in coexistence mode

Previously only ~20 shared paths were preserved. Now in coexistence mode
(claude_ccgs_coexist, codex_ccgs_coexist, multi_runtime), ALL existing
non-.opencode/ files at the target are preserved — including project-
specific files like README.md, LICENSE, .gitignore, and .github/*.

Only .opencode/ files (our exclusive runtime space) are written/updated,
plus AGENTS.md (marker-block splice for new files, append for existing).

Author: Cipher-85

.opencode/install.sh

@@ -262,8 +262,8 @@ for entry in data.get('files', []):
         skipped_foreign += 1
         continue
 
-    # Coexistence: preserve shared paths that already exist
-    if is_coexist_mode(mode) and is_shared(rel) and os.path.exists(dst):
+    # Coexistence: preserve ALL existing non-.opencode files (not just shared paths)
+    if is_coexist_mode(mode) and os.path.exists(dst) and not rel.startswith('.opencode/'):
         preserved_list.append(rel)
         preserved += 1
         continue