feat(agent): two-stage memory system with Dream consolidation

Replace single-stage MemoryConsolidator with a two-stage architecture:

- Consolidator: lightweight token-budget triggered summarization,
  appends to HISTORY.md with cursor-based tracking
- Dream: cron-scheduled two-phase processor that analyzes HISTORY.md
  and updates SOUL.md, USER.md, MEMORY.md via AgentRunner with
  edit_file tools for surgical, fault-tolerant updates

New files: MemoryStore (pure file I/O), Dream class, DreamConfig,
/dream and /dream-log commands. 89 tests covering all components.

tests/agent/test_consolidator.py

@@ -0,0 +1,78 @@
+"""Tests for the lightweight Consolidator — append-only to HISTORY.md."""
+
+import pytest
+import asyncio
+from unittest.mock import AsyncMock, MagicMock, patch
+
+from nanobot.agent.memory import Consolidator, MemoryStore
+
+
+@pytest.fixture
+def store(tmp_path):
+    return MemoryStore(tmp_path)
+
+
+@pytest.fixture
+def mock_provider():
+    p = MagicMock()
+    p.chat_with_retry = AsyncMock()
+    return p
+
+
+@pytest.fixture
+def consolidator(store, mock_provider):
+    sessions = MagicMock()
+    sessions.save = MagicMock()
+    return Consolidator(
+        store=store,
+        provider=mock_provider,
+        model="test-model",
+        sessions=sessions,
+        context_window_tokens=1000,
+        build_messages=MagicMock(return_value=[]),
+        get_tool_definitions=MagicMock(return_value=[]),
+        max_completion_tokens=100,
+    )
+
+
+class TestConsolidatorSummarize:
+    async def test_summarize_appends_to_history(self, consolidator, mock_provider, store):
+        """Consolidator should call LLM to summarize, then append to HISTORY.md."""
+        mock_provider.chat_with_retry.return_value = MagicMock(
+            content="User fixed a bug in the auth module."
+        )
+        messages = [
+            {"role": "user", "content": "fix the auth bug"},
+            {"role": "assistant", "content": "Done, fixed the race condition."},
+        ]
+        result = await consolidator.archive(messages)
+        assert result is True
+        entries = store.read_unprocessed_history(since_cursor=0)
+        assert len(entries) == 1
+
+    async def test_summarize_raw_dumps_on_llm_failure(self, consolidator, mock_provider, store):
+        """On LLM failure, raw-dump messages to HISTORY.md."""
+        mock_provider.chat_with_retry.side_effect = Exception("API error")
+        messages = [{"role": "user", "content": "hello"}]
+        result = await consolidator.archive(messages)
+        assert result is True  # always succeeds
+        entries = store.read_unprocessed_history(since_cursor=0)
+        assert len(entries) == 1
+        assert "[RAW]" in entries[0]["content"]
+
+    async def test_summarize_skips_empty_messages(self, consolidator):
+        result = await consolidator.archive([])
+        assert result is False
+
+
+class TestConsolidatorTokenBudget:
+    async def test_prompt_below_threshold_does_not_consolidate(self, consolidator):
+        """No consolidation when tokens are within budget."""
+        session = MagicMock()
+        session.last_consolidated = 0
+        session.messages = [{"role": "user", "content": "hi"}]
+        session.key = "test:key"
+        consolidator.estimate_session_prompt_tokens = MagicMock(return_value=(100, "tiktoken"))
+        consolidator.archive = AsyncMock(return_value=True)
+        await consolidator.maybe_consolidate_by_tokens(session)
+        consolidator.archive.assert_not_called()