fix: multi-user distillation + datetime in context + session log labels

Distillation was silently operating on scott/inara for all users due to
ContextVar defaults. All three distill endpoints now require ?user=&persona=
query params and validate them via persona.validate(). Memory distiller
signatures changed from Optional to required positional args — no more
global settings fallback. Scheduler now iterates all users/personas instead
of hardcoding the primary user.

- context_loader: inject current date/time as first system prompt section
- session_logger: use get_user()/get_persona() from context instead of
  settings globals so Holly/Brian sessions show correct speaker labels
- memory_distiller: system prompts now reference u.title()/p.title()
  instead of settings.user_name/settings.agent_name
- distill router: Query(...) enforces params; _resolve() validates persona
- scheduler: _all_personas() helper iterates every user/persona for distill
- app.js: runDistill() now appends ?user=&persona= via _fileParams

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

cortex/static/app.js

@@ -1724,7 +1724,7 @@
         async function runDistill(endpoint) {
             showDistillStatus('distilling…', false);
             try {
-                const res = await fetch(`/distill/${endpoint}`, { method: 'POST' });
+                const res = await fetch(`/distill/${endpoint}?${_fileParams}`, { method: 'POST' });
                 const d = await res.json();
                 if (!res.ok || d.ok === false) {
                     const err = d.error || d.mid?.error || d.long?.error || `HTTP ${res.status}`;