gateway: OpenCode (Zen + Go) provider adapter

Wires opencode.ai as a /v1/chat provider. One sk-* key reaches 40
models across Anthropic, OpenAI, Google, Moonshot, DeepSeek, Zhipu,
Alibaba, Minimax — billed against either the user's Zen balance
(pay-per-token premium models) or Go subscription (flat-rate
Kimi/GLM/DeepSeek/etc.). The unified /zen/v1 endpoint routes both;
upstream picks the billing tier based on model id.

Notable adapter quirks:

- Strip "opencode/" prefix on outbound (mirrors openrouter/kimi
  pattern). Caller can use {provider:"opencode", model:"X"} or
  {model:"opencode/X"}.
- Drop temperature for claude-*, gpt-5*, o1/o3/o4 models. Anthropic
  and OpenAI's reasoning lineage rejects temperature with 400
  "deprecated for this model". OCChatBody now serializes temperature
  as Option<f64> with skip_serializing_if so omitting it produces
  clean JSON.
- max_tokens.filter(|&n| n > 0) catches Some(0) — defensive after
  the same trap bit kimi.rs (empty env -> Number("") -> 0 -> 503).
- 600s default upstream timeout; reasoning models on big audit
  prompts legitimately take 3-5 min. Override OPENCODE_TIMEOUT_SECS.

Key handling:
- /etc/lakehouse/opencode.env (0600 root) loaded via systemd
  EnvironmentFile. Same pattern as kimi.env.
- OPENCODE_API_KEY env first, file scrape as fallback.

Verified end-to-end:
  opencode/claude-opus-4-7   -> "I'm Claude, made by Anthropic."
  opencode/kimi-k2.6         -> PONG-K26-GO
  opencode/deepseek-v4-pro   -> PONG-DS-V4
  opencode/glm-5.1           -> PONG-GLM
  opencode/minimax-m2.5-free -> PONG-FREE

Pricing reference (per audit @ ~14k in / 6k out):
  claude-opus-4-7   ~$0.22  (Zen)
  claude-haiku-4-5  ~$0.04  (Zen)
  gpt-5.5-pro       ~$1.50  (Zen)
  gemini-3-flash    ~$0.03  (Zen)
  kimi-k2.6 / glm / deepseek / qwen / minimax / mimo: covered by Go
  subscription ($10/mo, $60/mo cap).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config/providers.toml

@@ -45,6 +45,27 @@ default_model = "openai/gpt-oss-120b:free"
 # Model-prefix routing: "openrouter/<vendor>/<model>" auto-routes here,
 # prefix stripped before upstream call.
 
+[[provider]]
+name = "opencode"
+base_url = "https://opencode.ai/zen/v1"
+# Unified endpoint — covers BOTH Zen (pay-per-token Anthropic/OpenAI/
+# Gemini frontier) AND Go (flat-sub Kimi/GLM/DeepSeek/Qwen/Minimax).
+# Upstream bills per-model: Zen models hit Zen balance, Go models hit
+# Go subscription cap. /zen/go/v1 is the Go-only sub-path (rejects
+# Zen models), kept for reference but not used by this provider.
+auth = "bearer"
+auth_env = "OPENCODE_API_KEY"
+default_model = "claude-opus-4-7"
+# OpenCode (Zen + GO unified endpoint). One sk-* key reaches Claude
+# Opus 4.7, GPT-5.5-pro, Gemini 3.1-pro, Kimi K2.6, DeepSeek, GLM,
+# Qwen, plus 4 free-tier models. OpenAI-compatible Chat Completions
+# at /v1/chat/completions. Model-prefix routing: "opencode/<name>"
+# auto-routes here, prefix stripped before upstream call.
+# Key file: /etc/lakehouse/opencode.env (loaded via systemd EnvironmentFile).
+# Model catalog: curl -H "Authorization: Bearer ..." https://opencode.ai/zen/v1/models
+# Note: /zen/go/v1 is the GO-only sub-path (Kimi/GLM/DeepSeek tier);
+# /zen/v1 covers everything including Anthropic (which /zen/go/v1 rejects).
+
 [[provider]]
 name = "kimi"
 base_url = "https://api.kimi.com/coding/v1"