feat(coding-agent): expose post-compaction token estimates

closes #5877

packages/coding-agent/docs/rpc.md

@@ -374,11 +374,14 @@ Response:
     "summary": "Summary of conversation...",
     "firstKeptEntryId": "abc123",
     "tokensBefore": 150000,
+    "estimatedTokensAfter": 32000,
     "details": {}
   }
 }
 ```
 
+`estimatedTokensAfter` is a heuristic estimate over the rebuilt message context immediately after compaction, not a provider-exact token count.
+
 #### set_auto_compaction
 
 Enable or disable automatic compaction when context is nearly full.
@@ -924,6 +927,7 @@ The `reason` field is `"manual"`, `"threshold"`, or `"overflow"`.
     "summary": "Summary of conversation...",
     "firstKeptEntryId": "abc123",
     "tokensBefore": 150000,
+    "estimatedTokensAfter": 32000,
     "details": {}
   },
   "aborted": false,