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Preserve user guidance through history compression, stabilize Flash index targets, and retire withdrawn checks without applying stale verdicts.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

@@ -74,7 +74,7 @@ async def test_update_note_direct_execution_success(mock_ctx, tmp_path):
         target="- [ ] Step 1",
         replacement="- [x] Step 1",
     )
-    assert result == "Successfully updated note 'plan'."
+    assert result == "Updated note 'plan'."
 
     note_path = Path(tmp_path) / "notes" / "plan.md"
     assert note_path.read_text(encoding="utf-8") == "- [x] Step 1\n- [ ] Step 2"
@@ -95,7 +95,7 @@ async def test_update_note_with_warning(mock_ctx, tmp_path):
             target="Step 1",
             replacement="Step 1 Done",
         )
-        assert "Successfully updated note 'fuzzy_plan'." in result
+        assert "Updated note 'fuzzy_plan'." in result
         assert "WARNING: Fuzzy match applied" in result
 
 
@@ -110,7 +110,7 @@ async def test_update_note_callable_execution(mock_ctx, tmp_path):
         target="- [ ] Task 2",
         replacement="- [x] Task 2",
     )
-    assert result == "Successfully updated note 'callable_plan'."
+    assert result == "Updated note 'callable_plan'."
 
     note_path = Path(tmp_path) / "notes" / "callable_plan.md"
     assert note_path.read_text(encoding="utf-8") == "- [ ] Task 1\n- [x] Task 2"
@@ -134,7 +134,7 @@ async def test_update_note_with_state_tool_message(mock_ctx, tmp_path):
 
     assert isinstance(result, ToolMessage)
     assert result.tool_call_id == "call_update_999"
-    assert result.content == "Successfully updated note 'state_plan'."
+    assert result.content == "Updated note 'state_plan'."
     assert result.status == "success"
 
     note_path = Path(tmp_path) / "notes" / "state_plan.md"
@@ -168,7 +168,7 @@ async def test_get_update_note_tool_langchain_ainvoke(mock_ctx, tmp_path):
             "replacement": "delta",
         }
     )
-    assert result == "Successfully updated note 'lc_plan'."
+    assert result == "Updated note 'lc_plan'."
 
     note_path = Path(tmp_path) / "notes" / "lc_plan.md"
     assert note_path.read_text(encoding="utf-8") == "alpha delta gamma"
@@ -189,7 +189,7 @@ async def test_get_update_note_tool_pure(mock_ctx, tmp_path):
             "replacement": "Artemis",
         }
     )
-    assert result == "Successfully updated note 'pure_plan'."
+    assert result == "Updated note 'pure_plan'."
 
     note_path = Path(tmp_path) / "notes" / "pure_plan.md"
     assert note_path.read_text(encoding="utf-8") == "hello Artemis"