feat: TimescaleDB hypertable migration and README

Add hypertable migration that only runs on PostgreSQL (no-op for SQLite dev/tests).
Update README with architecture, quick start, and client sync instructions.

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

src/openclaw/migrations/0002_add_hypertables.py

@@ -0,0 +1,33 @@
+from django.db import migrations, connection
+
+CREATE_HYPERTABLES = """
+SELECT create_hypertable(
+    'sessions',
+    'start_time',
+    if_not_exists => TRUE
+);
+
+SELECT create_hypertable(
+    'messages',
+    'timestamp',
+    if_not_exists => TRUE
+);
+"""
+
+
+def create_hypertables(apps, schema_editor):
+    if connection.vendor != "postgresql":
+        return
+    with schema_editor.connection.cursor() as cursor:
+        cursor.execute(CREATE_HYPERTABLES)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("openclaw", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.RunPython(create_hypertables, migrations.RunPython.noop),
+    ]