fix(compressor): strip stale handoff prefix on resume; reconcile #26290+#32787 (#35344)

A handoff persisted under an older SUMMARY_PREFIX can be inherited into a
resumed lineage. _strip_summary_prefix only matched the current/legacy
literal, so on re-compaction the old 'resume exactly from Active Task'
directive stayed embedded in the body and kept hijacking replies to new,
unrelated user messages.

- Add _HISTORICAL_SUMMARY_PREFIXES (pre-#35344 prefix) and strip/recognize
  them in _strip_summary_prefix + _is_context_summary_content so resumed
  stale handoffs are re-normalized to the current latest-message-wins prefix.
- Reconcile the overlapping Active Task template edits from the salvaged
  #26290 (reverse-signal cancellation) and #32787 (capture open questions /
  decisions, don't write None too eagerly) — both intents kept.
- Regression coverage in tests/agent/test_resume_stale_active_task.py.
- AUTHOR_MAP entries for both salvaged contributors.

agent/context_compressor.py

@@ -61,6 +61,26 @@ SUMMARY_PREFIX = (
 )
 LEGACY_SUMMARY_PREFIX = "[CONTEXT SUMMARY]:"
 
+# Handoff prefixes that shipped in earlier releases. A summary persisted under
+# one of these can be inherited into a resumed lineage (#35344); when it is
+# re-normalized on re-compaction we must strip the OLD prefix too, otherwise the
+# stale directive it carried (e.g. "resume exactly from Active Task") survives
+# embedded in the body and keeps hijacking replies. Keep newest-first; entries
+# are matched literally. Add a frozen copy here whenever SUMMARY_PREFIX changes.
+_HISTORICAL_SUMMARY_PREFIXES = (
+    # Pre-#35344: contained the self-contradicting "resume exactly" directive.
+    "[CONTEXT COMPACTION — REFERENCE ONLY] Earlier turns were compacted "
+    "into the summary below. This is a handoff from a previous context "
+    "window — treat it as background reference, NOT as active instructions. "
+    "Do NOT answer questions or fulfill requests mentioned in this summary; "
+    "they were already addressed. "
+    "Your current task is identified in the '## Active Task' section of the "
+    "summary — resume exactly from there. "
+    "Respond ONLY to the latest user message "
+    "that appears AFTER this summary. The current session state (files, "
+    "config, etc.) may reflect work described here — avoid repeating it:",
+)
+
 # Minimum tokens for the summary output
 _MIN_SUMMARY_TOKENS = 2000
 # Proportion of compressed content to allocate for summary
@@ -1496,9 +1516,16 @@ The user has requested that this compaction PRIORITISE preserving all informatio
 
     @staticmethod
     def _strip_summary_prefix(summary: str) -> str:
-        """Return summary body without the current or legacy handoff prefix."""
+        """Return summary body without the current, legacy, or any historical
+        handoff prefix.
+
+        Historical prefixes must be stripped too: a handoff persisted under an
+        older prefix can be inherited into a resumed lineage (#35344), and if we
+        only re-prepend the current prefix without removing the old one, the
+        stale directive it carried stays embedded in the body.
+        """
         text = (summary or "").strip()
-        for prefix in (SUMMARY_PREFIX, LEGACY_SUMMARY_PREFIX):
+        for prefix in (SUMMARY_PREFIX, LEGACY_SUMMARY_PREFIX, *_HISTORICAL_SUMMARY_PREFIXES):
             if text.startswith(prefix):
                 return text[len(prefix):].lstrip()
         return text
@@ -1512,7 +1539,9 @@ The user has requested that this compaction PRIORITISE preserving all informatio
     @staticmethod
     def _is_context_summary_content(content: Any) -> bool:
         text = _content_text_for_contains(content).lstrip()
-        return text.startswith(SUMMARY_PREFIX) or text.startswith(LEGACY_SUMMARY_PREFIX)
+        if text.startswith(SUMMARY_PREFIX) or text.startswith(LEGACY_SUMMARY_PREFIX):
+            return True
+        return any(text.startswith(p) for p in _HISTORICAL_SUMMARY_PREFIXES)
 
     @classmethod
     def _find_latest_context_summary(