fix(chat): gate qwen3 thinking on message content instead of always-on

The frontend hardcoded think=true on every chat send (ChatPanel full +
widget variants, KnowledgeView minichat), which defeated the _should_think
gate on the backend and made qwen3:14b spend 5-20s on chain-of-thought
reasoning for every turn — even "hi". This was the root cause of the
warm-path TTFT variance tracked in followup_ttft_variance.md: the logged
ttft_ms was really prefill + full thinking phase, bouncing with the depth
of the model's reasoning, not with cache or eviction.

All three frontend callers now pass think=false and let _should_think be
authoritative. The classifier is now a real content-based gate: explicit
think_requested=True still forces on as an override (briefing discuss
actions, future UI toggles, MCP callers), otherwise messages <80 chars
without reasoning keywords skip thinking, messages >=400 chars or
containing keywords like why/explain/analyze/debug/review/etc. get it.

Generation timing now separately records think_requested, the final
think decision, first_token_ms (first any chunk), and thinking_ms
(duration of the thinking phase). ttft_ms keeps its existing semantic
(first content token) so existing log analysis still works. The timing
log line surfaces all four fields so the old "just a big ttft number"
ambiguity is gone.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ChatPanel.vue

@@ -208,7 +208,7 @@ async function onSubmit(payload: { content: string; contextNoteId?: number }) {
     payload.content,
     payload.contextNoteId,
     includedNoteIds.value.size ? [...includedNoteIds.value] : undefined,
-    true,
+    false,
     undefined,
     excludedNoteIds.value.length ? excludedNoteIds.value : undefined,
     props.projectId ?? store.ragProjectId,
@@ -249,7 +249,7 @@ async function widgetSend(payload: { content: string; contextNoteId?: number })
   widgetConvId.value = conv.id
   emit('conversation-started', conv.id)
 
-  await store.sendMessage(payload.content, payload.contextNoteId, undefined, true)
+  await store.sendMessage(payload.content, payload.contextNoteId, undefined, false)
 
   const msgs = store.currentConversation?.messages ?? []
   const lastAssistant = [...msgs].reverse().find((m: Message) => m.role === 'assistant')

frontend/src/views/KnowledgeView.vue

@@ -255,7 +255,7 @@ async function onMinichatSubmit(payload: { content: string; contextNoteId?: numb
   }
   chatOpen.value = true;
   chatCollapsed.value = false;
-  await chatStore.sendMessage(payload.content, payload.contextNoteId, undefined, true);
+  await chatStore.sendMessage(payload.content, payload.contextNoteId, undefined, false);
 }
 
 // ─── Auto-refresh cards when chat creates/edits notes or tasks ───────────────