Make balanced smart context option the default (#1186)

<!-- This is an auto-generated description by cubic. -->

## Summary by cubic
Set “balanced” as the default smart context mode. Users now get balanced
when Smart Files Context is enabled and no mode is set; “conservative”
must be explicitly selected.

- **Refactors**
- Default fallback to balanced in UI and engine (proSmartContextOption
undefined -> "balanced").
- ProModeSelector saves "conservative" explicitly; selector reads
undefined as balanced.
  - Updated schema and types to allow "balanced" | "conservative".
- Engine payload now includes smart_context_mode with "balanced" by
default; e2e tests and snapshots updated.

- **Migration**
- No action needed. Existing users without an explicit mode will use
balanced by default; selecting conservative persists.

<!-- End of auto-generated description by cubic. -->

e2e-tests/snapshots/context_manage.spec.ts_manage-context---smart-context---auto-includes-only-1.txt

@@ -98,7 +98,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/context_manage.spec.ts_manage-context---smart-context-1.txt

@@ -68,7 +68,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/context_manage.spec.ts_manage-context---smart-context-3.txt

@@ -56,7 +56,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": false
+      "enable_smart_files_context": false,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/context_manage.spec.ts_manage-context---smart-context-4.txt

@@ -114,7 +114,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": false
+      "enable_smart_files_context": false,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_regular-auto-should-send-message-to-engine-1.txt

@@ -413,7 +413,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": false
+      "enable_smart_files_context": false,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_send-message-to-engine---anthropic-claude-sonnet-4-1.txt

@@ -408,7 +408,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_send-message-to-engine---openai-gpt-5-1.txt

@@ -408,7 +408,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_send-message-to-engine---smart-context-conservative-1.txt

@@ -414,7 +414,7 @@
       ],
       "enable_lazy_edits": true,
       "enable_smart_files_context": true,
-      "smart_context_mode": "balanced"
+      "smart_context_mode": "conservative"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_send-message-to-engine-1.txt

@@ -413,7 +413,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/engine.spec.ts_smart-auto-should-send-message-to-engine-1.txt

@@ -413,7 +413,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/smart_context_options.spec.ts_switching-smart-context-mode-saves-the-right-setting-4.txt

@@ -19,6 +19,7 @@
   "lastShownReleaseNotesVersion": "[scrubbed]",
   "enableProLazyEditsMode": true,
   "enableProSmartFilesContextMode": true,
+  "proSmartContextOption": "conservative",
   "selectedTemplateId": "react",
   "selectedChatMode": "build",
   "enableAutoFixProblems": false,

e2e-tests/snapshots/thinking_budget.spec.ts_thinking-budget-2.txt

@@ -421,7 +421,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/thinking_budget.spec.ts_thinking-budget-4.txt

@@ -429,7 +429,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {

e2e-tests/snapshots/thinking_budget.spec.ts_thinking-budget-6.txt

@@ -437,7 +437,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {