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Preserve user guidance through history compression, stabilize Flash index targets, and retire withdrawn checks without applying stale verdicts.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

@@ -45,6 +45,7 @@ from artemis.config.agent import MemoryTranscriptConfig
 from artemis.context import ArtemisContext
 from artemis.memory.step_memory import StepMemoryService
 from artemis.memory.transcript import (
+    EPHEMERAL_BLOCKS_KEY,
     EXECUTION_RESULT_MARKER,
     PLAN_RECITATION_MARKER,
     RESTORED_HISTORY_HEADER,
@@ -53,8 +54,8 @@ from artemis.memory.transcript import (
 
 # SHA-256 snapshots of the legacy system message with the fixed inputs below.
 # Update these when an intentional template change alters the rendered prompt.
-GOLDEN_EMPTY_PLAN = "d8cf83565c766d32893b124c974060bbd98de2d862c3b9925e546ea0c946c324"
-GOLDEN_SENTINEL_PLAN = "8f871b5e5700ba5ee07ed03b80a8f82664abe68b0c170d3185bf1ee35f7821ed"
+GOLDEN_EMPTY_PLAN = "513e0341014b9a2342f608ad4dd173b7dcf5704ce53ee028ba302a9a1b696f96"
+GOLDEN_SENTINEL_PLAN = "012fe1b08645266d5a7e518023e20fc6fbfa65d4f2d985847ebb06e0184c67ee"
 
 SCREENSHOT_B64 = base64.b64encode(b"fake-jpeg-bytes").decode("utf-8")
 
@@ -103,6 +104,8 @@ def test_transcript_static_system_is_stable_and_carries_no_history():
     # ...and no unrendered slot or trailing observation header survives.
     assert "{{" not in first
     assert not first.endswith("# CURRENT OBSERVATION\n")
+    # The history guide names the marker a resolved screenshot renders as.
+    assert "--- Historical Visual Transition ---" in first
 
 
 def _transcript_ctx():
@@ -214,7 +217,9 @@ async def test_transcript_mode_two_turns_build_four_regions(tmp_path):
         # Committed turn-1 observation followed by its validator result.
         assert any(EXECUTION_RESULT_MARKER in str(m.content) for m in active)
         committed_obs = active[0]
-        # Depth-1 scrub removed the old plan recitation copy.
+        # Text edge at depth 1: the committed observation has lost its plan
+        # recitation as soon as the next tail exists (its screenshot stays
+        # until the screenshot edge).
         assert PLAN_RECITATION_MARKER not in str(committed_obs.content)
         # The live tail still recites the plan.
         assert PLAN_RECITATION_MARKER in str(turn2_messages[-1].content)
@@ -223,6 +228,52 @@ async def test_transcript_mode_two_turns_build_four_regions(tmp_path):
         assert re.search(r"T\+\d{2,}:\d{2}", str(turn2_messages[-1].content))
 
 
+@pytest.mark.asyncio
+async def test_transcript_tail_marks_per_turn_notices_ephemeral():
+    """The hand-built tail carries the builder's ephemeral indices: per-turn
+    notices are deleted by the scrub edge; header, plan recitation and the
+    observation itself are not."""
+    from artemis.agents.operator.prompts import USER_GUIDANCE_MARKER
+
+    ctx = _transcript_ctx()
+    captured: list = []
+    mock_llm = _no_action_llm(captured)
+    with patch("artemis.agents.operator.operator.get_llm", return_value=mock_llm):
+        node = OperatorNode(ctx, transcript_config=MemoryTranscriptConfig(enabled=True))
+        await node(
+            _transcript_state(
+                injected_instruction="Skip the login step",
+                user_stop_requested=False,
+                operator_tool_limit_exceeded=True,
+            )
+        )
+
+    tail = captured[0][-1]
+    assert isinstance(tail, HumanMessage)
+    texts = {
+        i: block.get("text", "")
+        for i, block in enumerate(tail.content)
+        if isinstance(block, dict) and block.get("type") == "text"
+    }
+    guidance = next(i for i, t in texts.items() if t.startswith(USER_GUIDANCE_MARKER))
+    warning = next(i for i, t in texts.items() if "tool-call budget" in t)
+    assert set(tail.additional_kwargs[EPHEMERAL_BLOCKS_KEY]) == {guidance, warning}
+    # The user's stop bit is relayed on the guidance block itself.
+    assert "authorized stopping" not in texts[guidance]
+    # The verbatim instruction is its own persistent block right after the
+    # wrapper (stamped with the ledger's session offset), never ephemeral:
+    # a standing instruction must outlive the turn it arrived on.
+    body = guidance + 1
+    assert re.fullmatch(r'User instruction \(T\+\d{2,}:\d{2}\): "Skip the login step"', texts[body])
+    assert body not in tail.additional_kwargs[EPHEMERAL_BLOCKS_KEY]
+    assert "Skip the login step" not in texts[guidance]
+    # Header, plan recitation and the observation stay in the history.
+    assert texts[0].startswith("# CURRENT OBSERVATION")
+    plan = next(i for i, t in texts.items() if t.startswith(PLAN_RECITATION_MARKER))
+    screenshot = next(i for i, t in texts.items() if t == "--- Current Screenshot ---")
+    assert not {0, plan, screenshot} & set(tail.additional_kwargs[EPHEMERAL_BLOCKS_KEY])
+
+
 @pytest.mark.asyncio
 async def test_transcript_actionless_turn_commits_without_validator_message():
     ctx = _transcript_ctx()