fix: tighten memory and session recall guidance

Remove diary-style memory framing from the system prompt and memory tool
schema, explicitly steer task/session logs to session_search, and clarify
that session_search is for cross-session recall after checking the current
conversation first. Add regression tests for the updated guidance text.

tests/agent/test_prompt_builder.py

@@ -15,10 +15,30 @@ from agent.prompt_builder import (
     build_context_files_prompt,
     CONTEXT_FILE_MAX_CHARS,
     DEFAULT_AGENT_IDENTITY,
+    MEMORY_GUIDANCE,
+    SESSION_SEARCH_GUIDANCE,
     PLATFORM_HINTS,
 )
 
 
+# =========================================================================
+# Guidance constants
+# =========================================================================
+
+
+class TestGuidanceConstants:
+    def test_memory_guidance_discourages_task_logs(self):
+        assert "durable facts" in MEMORY_GUIDANCE
+        assert "Do NOT save task progress" in MEMORY_GUIDANCE
+        assert "session_search" in MEMORY_GUIDANCE
+        assert "like a diary" not in MEMORY_GUIDANCE
+        assert ">80%" not in MEMORY_GUIDANCE
+
+    def test_session_search_guidance_is_simple_cross_session_recall(self):
+        assert "relevant cross-session context exists" in SESSION_SEARCH_GUIDANCE
+        assert "recent turns of the current session" not in SESSION_SEARCH_GUIDANCE
+
+
 # =========================================================================
 # Context injection scanning
 # =========================================================================