fix(opencode): the auth check is the binary, and create takes the adapter's verdict (Issue #60, Linux item 3)

opencode 2.x needs no login: a fresh HOME runs a free default model and a
local provider has no key, while 2.x keeps no auth.json. auth_ok is now
"opencode on PATH or under <HOME>/.opencode/bin". `python -m lib_py.agents
auth-ok <agent>` exits with an adapter's verdict, and create's opencode
branch uses it (same 7 lines) instead of its own auth.json/key test and
`opencode --version`, which wrote a line to the real opencode log.
test_a4's opencode block now pins the binary rule (decision D1, required
by the brief's item 3 and by both reviewers). Other agents are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-create/scripts/create_session.sh

@@ -143,9 +143,9 @@ elif [ "$AGENT" = "grok" ]; then
     exit 1
   fi
 elif [ "$AGENT" = "opencode" ]; then
-  if [ -f "$HOME/.local/share/opencode/auth.json" ] || [ -n "${ANTHROPIC_API_KEY:-}" ] || [ -n "${OPENAI_API_KEY:-}" ] || [ -n "${GEMINI_API_KEY:-}" ] || [ -n "${GROQ_API_KEY:-}" ]; then
-    true
-  elif ! opencode --version >/dev/null 2>&1; then
+  # opencode needs no login (2.x: a free default model, keyless local
+  # providers); the adapter's auth_ok is the one verdict.
+  if ! "$(_delegate_py_bin)" -m lib_py.agents auth-ok opencode >/dev/null 2>&1; then
     echo "ERROR: opencode is not functional or configured." >&2
     exit 1
   fi