feat: persist orchestrator sessions + user service + docs update

- Orchestrator now saves turns to session store so history survives page refresh
- UI session_id updated from job result; history controls attached to agent turns
- Cortex migrated from system service to systemd user service (no more sudo)
- Update README.md and CLAUDE.md with correct service commands

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

cortex/static/app.js

@@ -651,7 +651,8 @@
 
             activeController = new AbortController();
 
-            addMessage('user', text);
+            currentHistory.push({ role: 'user', content: text });
+            const userMsgDiv = addMessage('user', text);
             scrollToBottom();
 
             const thinkingDiv = addMessage('assistant thinking', '⚡ working…');
@@ -701,8 +702,20 @@
 
                 if (job.status === 'error') throw new Error(job.error || 'Orchestrator failed');
 
+                // Update session so this turn is part of the resumable history
+                if (job.session_id) {
+                    sessionId = job.session_id;
+                    sessionEl.textContent = `session: ${sessionId}`;
+                }
+
+                const userHistIdx = currentHistory.length - 1; // pushed before fetch
+                attachHistoryControls(userMsgDiv, userHistIdx);
+
                 thinkingDiv.className = 'message assistant';
                 setMessageText(thinkingDiv, 'assistant', job.response || '(no response)');
+                const assistHistIdx = currentHistory.length;
+                currentHistory.push({ role: 'assistant', content: job.response || '' });
+                attachHistoryControls(thinkingDiv, assistHistIdx);
 
                 const n = job.tool_calls?.length || 0;
                 if (n) {