fix(compression): guard against cross-session stale _previous_summary contamination

When a cron or background session compacts, it sets _previous_summary for
iterative updates. If that session ends without /new or /reset (which calls
on_session_reset()), the stale summary survives on the ContextCompressor
instance. A subsequent live messaging session's compaction then injects it as
'PREVIOUS SUMMARY:' into the summarizer prompt — contaminating the live
session with unrelated content from the prior session.

Add an else guard in compress(): when no handoff summary is found in the
current messages but _previous_summary is non-empty, discard it so
_generate_summary() starts fresh instead of iteratively updating a stale
cross-session summary.

Fixes #38788

agent/context_compressor.py

@@ -1990,6 +1990,13 @@ The user has requested that this compaction PRIORITISE preserving all informatio
             if summary_body and not self._previous_summary:
                 self._previous_summary = summary_body
             turns_to_summarize = messages[max(compress_start, summary_idx + 1):compress_end]
+        elif self._previous_summary:
+            # No handoff summary found in the current messages, but
+            # _previous_summary is non-empty — it was set by a different
+            # (now-ended) session (e.g., a cron job, a prior /new).  Discard
+            # it so _generate_summary() does not inject cross-session content
+            # into the summarizer prompt via the iterative-update path.
+            self._previous_summary = None
 
         if not self.quiet_mode:
             logger.info(