fix(opencode): use plural agents/ directory per current conventions

OpenCode docs now specify agents/ (plural) as the standard directory name;
agent/ (singular) is legacy backwards-compat. Update the converter output
path, installer source/dest paths, and .gitignore entry accordingly.

scripts/install.sh

@@ -306,8 +306,8 @@ install_gemini_cli() {
 }
 
 install_opencode() {
-  local src="$INTEGRATIONS/opencode/agent"
-  local dest="${PWD}/.opencode/agent"
+  local src="$INTEGRATIONS/opencode/agents"
+  local dest="${PWD}/.opencode/agents"
   local count=0
   [[ -d "$src" ]] || { err "integrations/opencode missing. Run convert.sh first."; return 1; }
   mkdir -p "$dest"