feat(qwen36-vl): split thinking — non-thinking default + qwen3.6-35b-a3b-thinking variant

The qwen3.6-35b-a3b VL checkpoint is a single hybrid model with a per-
request enable_thinking switch (Qwen3-style), defaulting thinking ON.
Make the default non-thinking and add an opt-in reasoning variant,
mirroring the existing glm-5.1 / glm-5.1-reasoning gateway split.

- qwen36-vl compose: add --reasoning-parser qwen3 (model-matched) so the
  single :8007 endpoint splits <think> into reasoning_content when on and
  routes all output to content when off — serving both modes cleanly.
- litellm gateway: base qwen3.6-35b-a3b pins chat_template_kwargs
  enable_thinking=false (non-thinking default); new qwen3.6-35b-a3b-thinking
  pins enable_thinking=true (opt-in reasoning). Same upstream checkpoint,
  no extra VRAM/container.

Deployed + verified on ana-ml2 (vLLM recreated, healthy) and ana-docker
(litellm reloaded): default returns a direct answer with no reasoning_content;
-thinking returns cleanly-separated reasoning_content, no raw tag leak.

stacks/qwen36-vl/compose.yaml

@@ -27,6 +27,18 @@
 # cost. 0.42 (~40 GB) = weights + graph + generous KV. Granite drops to 0.25/64K
 # to make room (the FP8-vs-maxed-granite tradeoff, operator-approved 2026-06-14).
 #
+# THINKING TOGGLE: this is ONE hybrid checkpoint (not separate Instruct/Thinking
+# downloads) with a Qwen3-style per-request `enable_thinking` switch. The chat
+# template defaults thinking ON (`<think>\n`); passing
+# `chat_template_kwargs={"enable_thinking":false}` emits the empty
+# `<think>\n\n</think>\n\n` block (no reasoning). We run --reasoning-parser qwen3
+# (model-matched — its vLLM docstring describes THIS checkpoint) so ONE endpoint
+# serves BOTH modes cleanly: thinking-ON splits <think>…</think> into
+# reasoning_content; thinking-OFF routes everything to content. The gateway
+# selects the mode per model_name (stacks/litellm/conf/config.yaml):
+#   qwen3.6-35b-a3b          → enable_thinking:false  (non-thinking DEFAULT)
+#   qwen3.6-35b-a3b-thinking → enable_thinking:true   (opt-in reasoning)
+#
 # All tunables live in .env — edit that, not this file.
 
 name: qwen36-vl
@@ -72,6 +84,13 @@ services:
       - --dtype
       - auto
       - --enable-prefix-caching
+      # Model-matched reasoning parser for the hybrid thinking toggle (see header).
+      # Splits <think>…</think> into reasoning_content when thinking is ON; routes
+      # all output to content when the empty think-block signals thinking OFF — so
+      # this single :8007 endpoint serves both the non-thinking default and the
+      # qwen3.6-35b-a3b-thinking gateway variant.
+      - --reasoning-parser
+      - qwen3
     deploy:
       resources:
         reservations: