refactor: unify agent history and harden runtime diagnostics

Preserve user guidance through history compression, stabilize Flash index targets, and retire withdrawn checks without applying stale verdicts.

Improve runtime diagnostics, tool responses, and history rendering. Simplify documentation and comments.

Validation: 2076 tests passed in the full suite; 381 affected tests and 26 frontend tests passed after cleanup. Ruff and the quality ratchet passed.

tests/unit/tools/test_append_note_tool.py

@@ -70,7 +70,7 @@ async def test_append_note_direct_execution_success(mock_ctx, tmp_path):
         key="test_append",
         content="First line",
     )
-    assert result1 == "Successfully appended note to test_append.md."
+    assert result1 == "Appended to note 'test_append'."
 
     note_path = Path(tmp_path) / "notes" / "test_append.md"
     assert note_path.exists()
@@ -82,7 +82,7 @@ async def test_append_note_direct_execution_success(mock_ctx, tmp_path):
         key="test_append",
         content="Second line",
     )
-    assert result2 == "Successfully appended note to test_append.md."
+    assert result2 == "Appended to note 'test_append'."
     assert note_path.read_text(encoding="utf-8") == "First line\nSecond line\n"
 
 
@@ -94,7 +94,7 @@ async def test_append_note_callable_execution(mock_ctx, tmp_path):
         key="callable_append",
         content="Callable append test",
     )
-    assert result == "Successfully appended note to callable_append.md."
+    assert result == "Appended to note 'callable_append'."
 
     note_path = Path(tmp_path) / "notes" / "callable_append.md"
     assert note_path.exists()
@@ -116,7 +116,7 @@ async def test_append_note_with_state_tool_message(mock_ctx, tmp_path):
 
     assert isinstance(result, ToolMessage)
     assert result.tool_call_id == "call_append_999"
-    assert result.content == "Successfully appended note to state_append_note.md."
+    assert result.content == "Appended to note 'state_append_note'."
     assert result.status == "success"
 
     note_path = Path(tmp_path) / "notes" / "state_append_note.md"
@@ -151,7 +151,7 @@ async def test_get_append_note_tool_langchain_ainvoke(mock_ctx, tmp_path):
             "content": "Appended via LangChain BaseTool",
         }
     )
-    assert result == "Successfully appended note to lc_append_note.md."
+    assert result == "Appended to note 'lc_append_note'."
 
     note_path = Path(tmp_path) / "notes" / "lc_append_note.md"
     assert note_path.exists()
@@ -170,7 +170,7 @@ async def test_get_append_note_tool_pure(mock_ctx, tmp_path):
             "content": "Appended via pure tool",
         }
     )
-    assert result == "Successfully appended note to pure_append_note.md."
+    assert result == "Appended to note 'pure_append_note'."
 
     note_path = Path(tmp_path) / "notes" / "pure_append_note.md"
     assert note_path.exists()