core/memory+cli: session rename, delete, and export

Add MemoryStore rename/delete with explicit message cleanup. Slash
commands /rename, /delete (confirm), /export md|json write transcripts
from DB; deleting the current session starts a fresh empty one.

tests/test_cli/test_export.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from plyngent.cli.export import (
+    encode_session_export_json,
+    format_session_export_md,
+    session_export_payload,
+)
+from plyngent.lmproto.openai_compatible.model import AssistantChatMessage, UserChatMessage
+
+
+def test_format_session_export_md() -> None:
+    text = format_session_export_md(
+        [
+            UserChatMessage(content="hi"),
+            AssistantChatMessage(content="yo", reasoning_content="plan"),
+        ],
+        sid=7,
+        name="demo",
+        workspace="/tmp/ws",
+    )
+    assert "# Session 7: demo" in text
+    assert "## user" in text
+    assert "hi" in text
+    assert "### reasoning" in text
+    assert "plan" in text
+    assert "yo" in text
+
+
+def test_session_export_json_roundtrip() -> None:
+    messages = [UserChatMessage(content="a")]
+    now = datetime.now(UTC)
+    payload = session_export_payload(
+        sid=7,
+        name="demo",
+        workspace="/tmp/ws",
+        created_at=now,
+        updated_at=now,
+        messages=messages,
+    )
+    assert isinstance(payload, dict)
+    raw = encode_session_export_json(payload)
+    assert "7" in raw
+    assert "demo" in raw