feat(command): add /history command to show recent session messages

Adds /history [n] to display the last N user/assistant messages from
the current session (default 10, max 50).

- Tool and system messages are filtered out for readability
- Long messages are truncated to 200 characters with an ellipsis
- Multimodal content (image blocks) is collapsed to its text parts
- Invalid count argument returns a usage hint
- /history n uses prefix routing; /history uses exact routing

Also registers /history in build_help_text().

tests/cli/test_restart_command.py

@@ -243,6 +243,58 @@ class TestRestartCommand:
         assert "Context: 1k/65k (1% of input budget)" in response.content
         assert "Tasks: 0 active" in response.content
 
+    @pytest.mark.asyncio
+    async def test_history_shows_recent_messages(self):
+        loop, _bus = _make_loop()
+        session = MagicMock()
+        session.get_history.return_value = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi there!"},
+            {"role": "tool", "content": "tool result"},  # should be filtered out
+            {"role": "user", "content": "How are you?"},
+            {"role": "assistant", "content": "I am doing well."},
+        ]
+        loop.sessions.get_or_create.return_value = session
+
+        msg = InboundMessage(channel="telegram", sender_id="u1", chat_id="c1", content="/history")
+        response = await loop._process_message(msg)
+
+        assert response is not None
+        assert "👤 You: Hello" in response.content
+        assert "🤖 Bot: Hi there!" in response.content
+        assert "tool result" not in response.content  # tool messages filtered
+        assert response.metadata == {"render_as": "text"}
+
+    @pytest.mark.asyncio
+    async def test_history_respects_count_argument(self):
+        loop, _bus = _make_loop()
+        session = MagicMock()
+        session.get_history.return_value = [
+            {"role": "user", "content": f"message {i}"} for i in range(20)
+        ]
+        loop.sessions.get_or_create.return_value = session
+
+        msg = InboundMessage(channel="telegram", sender_id="u1", chat_id="c1", content="/history 3")
+        response = await loop._process_message(msg)
+
+        assert response is not None
+        assert "Last 3 message(s)" in response.content
+        assert "message 19" in response.content  # most recent
+        assert "message 0" not in response.content  # too old
+
+    @pytest.mark.asyncio
+    async def test_history_empty_session(self):
+        loop, _bus = _make_loop()
+        session = MagicMock()
+        session.get_history.return_value = []
+        loop.sessions.get_or_create.return_value = session
+
+        msg = InboundMessage(channel="telegram", sender_id="u1", chat_id="c1", content="/history")
+        response = await loop._process_message(msg)
+
+        assert response is not None
+        assert "No conversation history yet." in response.content
+
     @pytest.mark.asyncio
     async def test_process_direct_preserves_render_metadata(self):
         loop, _bus = _make_loop()