feat(opencode-zen): add OpenCode Zen (PAYG) provider with free-tier fingerprint, alias ocz

Multi-endpoint provider on https://opencode.ai/zen/v1 (openai / claude /
openai-responses transports mirroring opencode-go) with 71 models across
paid + free tiers. Free-tier fingerprint: opencode/1.18.x UA spoof,
ses_-session header, built-in tool quartet, forced stream. Usage endpoint
/zen/v1/usage wired into the dashboard.

tests/__baseline__/alias-baseline.json

@@ -180,6 +180,7 @@
     "openai": "openai",
     "opencode": "oc",
     "opencode-go": "opencode-go",
+    "opencode-zen": "ocz",
     "openrouter": "openrouter",
     "perplexity": "perplexity",
     "perplexity-agent": "perplexity-agent",
@@ -265,6 +266,7 @@
     "nebius",
     "nvidia",
     "oc",
+    "ocz",
     "ollama",
     "openai",
     "openai-tts-models",