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>

crates/gateway/src/main.rs

@@ -283,6 +283,18 @@ async fn main() {
                 }
                 k
             },
+            opencode_key: {
+                // OpenCode GO multi-vendor gateway — Claude Opus 4.7,
+                // GPT-5.5-pro, Gemini 3.1-pro, Kimi K2.6, DeepSeek, GLM,
+                // Qwen + free-tier. Key from /etc/lakehouse/opencode.env.
+                let k = v1::opencode::resolve_opencode_key();
+                if k.is_some() {
+                    tracing::info!("v1: OpenCode key loaded — /v1/chat provider=opencode enabled (40 models)");
+                } else {
+                    tracing::debug!("v1: no OpenCode key — provider=opencode will 503");
+                }
+                k
+            },
             // Phase 40 early deliverable — Langfuse trace emitter.
             // Defaults match mcp-server/tracing.ts conventions so
             // gateway traces land in the same staffing project.