fix(gateway): persist transcript changes in /retry, /undo and fix /reset

/retry and /undo set session_entry.conversation_history which does not
exist on SessionEntry. The truncated history was never written to disk,
so the next message reload picked up the full unmodified transcript.

Added SessionStore.rewrite_transcript() that persists changes to both
the JSONL file and SQLite database, and updated both commands to use it.

/reset accessed self.session_store._sessions which does not exist on
SessionStore (the correct attribute is _entries). Also replaced the
hand-coded session key with _generate_session_key() to fix WhatsApp DM
sessions using the wrong key format.

Closes #210

tests/gateway/test_session.py

@@ -1,9 +1,13 @@
 """Tests for gateway session management."""
 
+import json
 import pytest
+from pathlib import Path
+from unittest.mock import patch, MagicMock
 from gateway.config import Platform, HomeChannel, GatewayConfig, PlatformConfig
 from gateway.session import (
     SessionSource,
+    SessionStore,
     build_session_context,
     build_session_context_prompt,
 )
@@ -199,3 +203,59 @@ class TestBuildSessionContextPrompt:
         prompt = build_session_context_prompt(ctx)
 
         assert "WhatsApp" in prompt or "whatsapp" in prompt.lower()
+
+
+class TestSessionStoreRewriteTranscript:
+    """Regression: /retry and /undo must persist truncated history to disk."""
+
+    @pytest.fixture()
+    def store(self, tmp_path):
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            s = SessionStore(sessions_dir=tmp_path, config=config)
+        s._db = None  # no SQLite for these tests
+        s._loaded = True
+        return s
+
+    def test_rewrite_replaces_jsonl(self, store, tmp_path):
+        session_id = "test_session_1"
+        # Write initial transcript
+        for msg in [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "hi"},
+            {"role": "user", "content": "undo this"},
+            {"role": "assistant", "content": "ok"},
+        ]:
+            store.append_to_transcript(session_id, msg)
+
+        # Rewrite with truncated history
+        store.rewrite_transcript(session_id, [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "hi"},
+        ])
+
+        reloaded = store.load_transcript(session_id)
+        assert len(reloaded) == 2
+        assert reloaded[0]["content"] == "hello"
+        assert reloaded[1]["content"] == "hi"
+
+    def test_rewrite_with_empty_list(self, store):
+        session_id = "test_session_2"
+        store.append_to_transcript(session_id, {"role": "user", "content": "hi"})
+
+        store.rewrite_transcript(session_id, [])
+
+        reloaded = store.load_transcript(session_id)
+        assert reloaded == []
+
+
+class TestSessionStoreEntriesAttribute:
+    """Regression: /reset must access _entries, not _sessions."""
+
+    def test_entries_attribute_exists(self):
+        config = GatewayConfig()
+        with patch("gateway.session.SessionStore._ensure_loaded"):
+            store = SessionStore(sessions_dir=Path("/tmp"), config=config)
+        store._loaded = True
+        assert hasattr(store, "_entries")
+        assert not hasattr(store, "_sessions")