Add sender context mode for text generation (#104)

Add a per-room/global `sender_context_mode` setting that optionally
prefixes conversation messages with sender metadata before sending them
to the model provider.

This helps models distinguish between participants in multi-user rooms.
                                                                                                                                                                                                                                                             
Three modes are supported:
- `disabled` (default, no change)
- `matrix_user_id` (prefixes with `[sender=@user:server]`)
- `matrix_user_id_and_timestamp` (adds `send_at`; Example: `[sender=@user:server sent_at=<ISO 8601>]`)
                                                                                                                                                                                                                                                             
Sender context is applied to user and assistant text messages only,
skipping system prompts and non-text content.

Mixed-sender merged turns (something we intentionally do for Anthropic)
have their `sender_id` cleared to avoid misattribution.

src/controller/cfg/dispatching/text_generation.rs

@@ -1,5 +1,6 @@
 use crate::entity::roomconfig::{
     RoomSettings, TextGenerationAutoUsage, TextGenerationPrefixRequirementType,
+    TextGenerationSenderContextMode,
 };
 use crate::{Bot, entity::MessageContext};
 
@@ -151,5 +152,38 @@ pub(super) async fn dispatch(
                 }
             }
         }
+
+        ConfigTextGenerationSettingRelatedControllerType::GetSenderContextMode => {
+            let value = &room_settings.text_generation.sender_context_mode;
+            setting_get::<TextGenerationSenderContextMode>(bot, message_context, value).await
+        }
+        ConfigTextGenerationSettingRelatedControllerType::SetSenderContextMode(value) => {
+            let value = value.to_owned();
+
+            let setter_callback = Box::new(move |room_settings: &mut RoomSettings| {
+                room_settings.text_generation.sender_context_mode = value;
+            });
+
+            match config_type {
+                SettingsStorageSource::Room => {
+                    room_setting_set::<TextGenerationSenderContextMode>(
+                        bot,
+                        message_context,
+                        &value,
+                        setter_callback,
+                    )
+                    .await
+                }
+                SettingsStorageSource::Global => {
+                    global_setting_set::<TextGenerationSenderContextMode>(
+                        bot,
+                        message_context,
+                        &value,
+                        setter_callback,
+                    )
+                    .await
+                }
+            }
+        }
     }
 }