fix(agent): freeze carveout-era SUMMARY_PREFIX for renormalization

The prompt consolidation above retires the carveout-era prefix. Without a
frozen copy in _HISTORICAL_SUMMARY_PREFIXES, summaries persisted by
pre-upgrade builds would lose detection (_is_context_summary_content) and
renormalization (_strip_summary_prefix) — the exact regression class the
tuple exists to prevent. Adds contract tests covering every frozen prefix.

Refs #41607 #38364 #42812

tests/agent/test_summary_prefix_semantics.py

@@ -80,3 +80,37 @@ def test_memory_authority_preserved():
     assert "MEMORY.md" in SUMMARY_PREFIX
     assert "USER.md" in SUMMARY_PREFIX
     assert "authoritative" in SUMMARY_PREFIX
+
+
+def test_no_background_consistency_carveout():
+    """The "consistent → use as background" carveout licensed stale-task
+    resumption on topic overlap (#41607, #38364, #42812). It must stay gone,
+    and the prefix must explicitly neutralize topic overlap."""
+    lower = SUMMARY_PREFIX.lower()
+    assert "you may use the summary as background" not in lower
+    assert "topic overlap" in lower
+
+
+def test_replaced_prefixes_are_frozen_for_renormalization():
+    """Every retired SUMMARY_PREFIX must be frozen into
+    _HISTORICAL_SUMMARY_PREFIXES, otherwise summaries persisted by older
+    builds lose detection/renormalization after an upgrade. The carveout-era
+    prefix is the latest retiree."""
+    from agent.context_compressor import (
+        _HISTORICAL_SUMMARY_PREFIXES,
+        ContextCompressor,
+    )
+
+    carveout_era = [
+        p for p in _HISTORICAL_SUMMARY_PREFIXES
+        if "you may use the summary as background" in p
+    ]
+    assert carveout_era, "carveout-era prefix missing from frozen tuple"
+    # The live prefix must never be one of the frozen ones.
+    assert SUMMARY_PREFIX not in _HISTORICAL_SUMMARY_PREFIXES
+    # Detection + strip must work for every frozen prefix.
+    for old_prefix in _HISTORICAL_SUMMARY_PREFIXES:
+        content = old_prefix + "\n## Summary body"
+        assert ContextCompressor._is_context_summary_content(content)
+        stripped = ContextCompressor._strip_summary_prefix(content)
+        assert not stripped.startswith(old_prefix)