docs: show populated metadata exemplar in compaction event examples

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Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

Author: colombod

docs/contracts/CONTEXT_CONTRACT.md

@@ -374,7 +374,7 @@ async def _compact_internal(self) -> None:
     await self._hooks.emit("context:pre_compact", {
         "message_count": len(self._messages),
         "token_count": self._token_count,
-        "metadata": None,  # Optional dict for implementation-specific data
+        "metadata": {"strategy": "sliding_window", "trigger": "token_limit", "budget": 195000},
     })
 
     # Build tool_call_id -> tool_use index map
@@ -417,7 +417,7 @@ async def _compact_internal(self) -> None:
     await self._hooks.emit("context:post_compact", {
         "message_count": len(self._messages),
         "token_count": self._token_count,
-        "metadata": None,  # Optional dict for implementation-specific data
+        "metadata": {"budget_remaining": 97500, "storage_quota_pct": 0.42},
     })
 ```