infra: replace gpt-oss with Ollama Pro + OpenCode Zen across hot paths

Ollama Pro plan went live today (39-model fleet on the same
OLLAMA_CLOUD_KEY) and OpenCode Zen was already wired in the gateway
but not consumed. Routing every gpt-oss call site to faster /
stronger replacements:

| Site | gpt-oss → replacement | Why |
|---|---|---|
| ollama_cloud default | gpt-oss:120b → deepseek-v3.2 | newest DeepSeek revision; live-probed `pong` |
| openrouter default | openai/gpt-oss-120b:free → x-ai/grok-4.1-fast | already the scrum LADDER's PRIMARY |
| modes.toml staffing_inference | openai/gpt-oss-120b:free → kimi-k2.6 | coding-specialized, on Ollama Pro |
| modes.toml doc_drift_check | gpt-oss:120b → gemini-3-flash-preview | speed leader for factual checks |
| scrum_master_pipeline tree-split MAP+REDUCE | gpt-oss:120b → gemini-3-flash-preview | latency-dominated path (5-20× per file) |
| bot/propose.ts CLOUD_MODEL | gpt-oss:120b → deepseek-v3.2 | same Ollama key, faster |
| mcp-server/observer.ts overseer label fallback | gpt-oss:120b → claude-opus-4-7 | matches new overseer model |
| crates/gateway/src/execution_loop overseer escalation | ollama_cloud/gpt-oss:120b → opencode/claude-opus-4-7 | frontier reasoning matters here — fires only after local self-correct fails twice; Zen pay-per-token cost is bounded |

Verification:
- `cargo check -p gateway --tests` — clean
- Live probes through localhost:3100/v1/chat:
  - `opencode/claude-opus-4-7` → "pong"
  - `gemini-3-flash-preview` (ollama_cloud) → "pong"
  - `kimi-k2.6` (ollama_cloud) → "pong"
  - `deepseek-v3.2` (ollama_cloud) → "Pong! 🏓"

Notes:
- kimi-k2:1t still upstream-broken (HTTP 500 on Ollama Pro probe today,
  matches yesterday's memory). Replacement table never picks it.
- The Rust changes need a `systemctl restart lakehouse.service` to
  take effect on the running gateway. TS callers reload on next run.
- aibridge/src/context.rs still has gpt-oss:{20b,120b} in its window-
  size lookup table; harmless and kept for callers that pass it
  explicitly as an override.

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

tests/real-world/scrum_master_pipeline.ts

@@ -1143,9 +1143,15 @@ Format each as a code-fenced block with the byte offset within the shard:
 EXACT LINE OF SOURCE — DO NOT PARAPHRASE, DO NOT TRUNCATE
 \`\`\`
 Pick the most reviewer-relevant lines: route definitions (e.g. \`@app.route(...)\`), function signatures, security-sensitive calls (auth/SQL/exec/template/secrets), hardcoded credentials/defaults, exception handlers, sensitive imports. The reviewer will REFUSE to act on any claim not backed by a verbatim anchor — so anchors are how you prove findings are real.`;
+    // 2026-04-28: gpt-oss:120b → gemini-3-flash-preview via Ollama
+    // Pro. Tree-split MAP fires once per shard (potentially 5-20×
+    // per file), so latency dominates total scrum time. Gemini 3
+    // flash returns shard digests substantially faster than the old
+    // 120B free model while staying strong enough for byte-anchored
+    // extraction.
     const r = await chat({
       provider: "ollama_cloud",
-      model: "gpt-oss:120b",
+      model: "gemini-3-flash-preview",
       prompt,
       max_tokens: 900,
     });
@@ -1195,9 +1201,14 @@ COPY EVERY anchor block from the piece notes IN ORDER, character-perfect. DO NOT
 
 Output the anchor blocks under their original \`\`\`@offset...\`\`\` fences, each on its own with a blank line between. The reviewer rejects findings that don't quote a string from this anchors block, so completeness here directly determines review quality.`;
 
+  // 2026-04-28: gpt-oss:120b → gemini-3-flash-preview via Ollama
+  // Pro. The reducer runs once per file (vs once per shard for MAP)
+  // but on a much larger context (all shard digests stacked), so
+  // throughput per token still matters. Same model as MAP for
+  // consistency in tree-split outputs.
   const reduced = await chat({
     provider: "ollama_cloud",
-    model: "gpt-oss:120b",
+    model: "gemini-3-flash-preview",
     prompt: reducePrompt,
     max_tokens: 2400,
   });