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.

agent/prompt_builder.py

@@ -71,15 +71,17 @@ DEFAULT_AGENT_IDENTITY = (
 )
 
 MEMORY_GUIDANCE = (
-    "You have persistent memory across sessions. Proactively save important things "
-    "you learn (user preferences, environment details, useful approaches) and do "
-    "(like a diary!) using the memory tool -- don't wait to be asked."
+    "You have persistent memory across sessions. Save durable facts using the memory "
+    "tool: user preferences, environment details, tool quirks, and stable conventions. "
+    "Memory is injected into every turn, so keep it compact. Do NOT save task progress, "
+    "session outcomes, or completed-work logs to memory; use session_search to recall "
+    "those from past transcripts."
 )
 
 SESSION_SEARCH_GUIDANCE = (
     "When the user references something from a past conversation or you suspect "
-    "relevant prior context exists, use session_search to recall it before asking "
-    "them to repeat themselves."
+    "relevant cross-session context exists, use session_search to recall it before "
+    "asking them to repeat themselves."
 )
 
 SKILLS_GUIDANCE = (