feat(compression): summaries now respect the conversation's language

Context compaction summaries were always produced in English regardless
of the conversation language, which injected English context into
non-English conversations and muddied the continuation experience.

Adds a one-sentence instruction to the shared `_summarizer_preamble`
used by both the initial-compaction and iterative-update prompt paths.
Placing it in the preamble (rather than adding it separately to each
prompt) means both code paths stay in sync with one edit.

Ported from anomalyco/opencode#20581. The original PR (#4670) landed
before main's prompt templates were refactored to share the
`_summarizer_preamble` and `_template_sections` blocks, so the
cherry-pick conflicted on the now-obsolete inline sections; re-applied
the essential one-line change on top of the current structure.

Verified: 48/48 existing compressor tests pass.

Author: teknium1

agent/context_compressor.py

@@ -633,7 +633,9 @@ def _generate_summary(self, turns_to_summarize: List[Dict[str, Any]], focus_topi
             "assistant that continues the conversation. "
             "Do NOT respond to any questions or requests in the conversation — "
             "only output the structured summary. "
-            "Do NOT include any preamble, greeting, or prefix."
+            "Do NOT include any preamble, greeting, or prefix. "
+            "Write the summary in the same language the user was using in the "
+            "conversation — do not translate or switch to English."
         )
 
         # Shared structured template (used by both paths).