llm/ollama: disable thinking so reasoning models return content

Qwen 3.5 — the spec's recommended model — is a reasoning model. With
thinking on, Ollama streams its chain-of-thought into a separate
`thinking` field and hits num_predict before emitting any answer into
`content`, so Complete() got an empty string and the checkpoint failed
with "no JSON object in model output". Sending `"think": false` on every
/api/chat request fixes it; non-thinking models (qwen2.5) ignore the flag.

Validated end-to-end on deployment hardware (Ollama, qwen3.5:9b): the
grammar checkpoint now caught all five ESL errors in a 3-sentence sample
with correct JSON and string-anchoring, ~8.5s warm.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

internal/llm/ollama.go

@@ -16,10 +16,17 @@ type OllamaClient struct {
 
 // ollamaRequest mirrors Ollama's /api/chat body. Sampling parameters live under
 // "options" (see the cheatsheet in the spec).
+//
+// Think is sent false unconditionally: reasoning models (e.g. qwen3.5) otherwise
+// stream their chain-of-thought into a separate "thinking" field and can exhaust
+// num_predict before emitting any answer in "content" — leaving us an empty
+// response. We want direct output (structured JSON for the checkpoint, concise
+// prose for chat), not reasoning. Non-thinking models simply ignore the flag.
 type ollamaRequest struct {
 	Model    string        `json:"model"`
 	Messages []Message     `json:"messages"`
 	Stream   bool          `json:"stream"`
+	Think    bool          `json:"think"`
 	Options  ollamaOptions `json:"options"`
 }