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>

tests/test_a4_adapter_contract.py

@@ -311,16 +311,24 @@ def test_adapter_auth_ok(tmp_path, monkeypatch):
     # Hermes always True
     assert get_adapter('hermes').auth_ok() is True
 
-    # OpenCode auth check (provider env var or auth.json file)
+    # OpenCode auth check: the binary alone (2.x needs no login, Issue #60);
+    # keys and auth.json do not count
     opencode = get_adapter('opencode')
+    path = os.environ["PATH"]
+    monkeypatch.setenv("PATH", str(tmp_path / "empty-bin"))
     assert opencode.auth_ok() is False
     monkeypatch.setenv("ANTHROPIC_API_KEY", "test-key")
-    assert opencode.auth_ok() is True
-    monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
     opencode_auth = tmp_path / ".local" / "share" / "opencode" / "auth.json"
     opencode_auth.parent.mkdir(parents=True, exist_ok=True)
     opencode_auth.write_text("{}")
+    assert opencode.auth_ok() is False
+    monkeypatch.delenv("ANTHROPIC_API_KEY", raising=False)
+    opencode_bin = tmp_path / ".opencode" / "bin" / "opencode"
+    opencode_bin.parent.mkdir(parents=True)
+    opencode_bin.write_text("#!/bin/sh\n")
+    opencode_bin.chmod(0o755)
     assert opencode.auth_ok() is True
+    monkeypatch.setenv("PATH", path)
 
     # Codex auth check (env var, auth.json file, or CLI)
     codex = get_adapter('codex')