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>

config/modes.toml

@@ -44,7 +44,10 @@ name = "staffing_inference"
 # pattern generalizes beyond code review.
 preferred_mode = "staffing_inference_lakehouse"
 fallback_modes = ["ladder", "consensus", "pipeline"]
-default_model = "openai/gpt-oss-120b:free"
+# 2026-04-28: gpt-oss-120b:free → kimi-k2.6 via Ollama Pro. Coding-
+# specialized, faster than gpt-oss, on the same OLLAMA_CLOUD_KEY so
+# no extra provider hop.
+default_model = "kimi-k2.6"
 matrix_corpus = "workers_500k_v8"
 
 [[task_class]]
@@ -58,7 +61,9 @@ matrix_corpus = "kb_team_runs_v1"
 name = "doc_drift_check"
 preferred_mode = "drift"
 fallback_modes = ["validator"]
-default_model = "gpt-oss:120b"
+# 2026-04-28: gpt-oss:120b → gemini-3-flash-preview via Ollama Pro.
+# Speed leader on factual checking, same OLLAMA_CLOUD_KEY.
+default_model = "gemini-3-flash-preview"
 matrix_corpus = "distilled_factual_v20260423095819"
 
 [[task_class]]