feat: reasoning token budget + session name in header

- reasoning_budget_tokens: optional int field on local_openai models;
  when set, injects {"reasoning": {"budget_tokens": N}} via extra_body
  into every OpenRouter API call (both tool-loop and confirmation-gate
  rounds). Field exposed in the model edit form in Settings.

- session name moved from standalone full-row div between #messages
  and #input-area into the persona-switcher block in the header, as a
  third dim line under "Cortex · Local". Collapses when empty via
  :empty CSS. No JS changes required.

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

cortex/static/style.css

@@ -897,11 +897,14 @@
         #stop:hover { background: #5c1a1a; }
 
         #session-id {
-            font-size: 0.7rem;
+            font-size: 0.68rem;
             color: var(--border);
-            padding: 0 20px 6px;
-            background: var(--surface);
+            white-space: nowrap;
+            overflow: hidden;
+            text-overflow: ellipsis;
+            max-width: 220px;
         }
+        #session-id:empty { display: none; }
 
         /* ── Message wrappers (edit/delete controls) ──────────────── */
         .msg-wrapper {