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/determination/text_generation/mod.rs

@@ -3,7 +3,10 @@ mod tests;
 
 use crate::{
     controller::ControllerType,
-    entity::roomconfig::{TextGenerationAutoUsage, TextGenerationPrefixRequirementType},
+    entity::roomconfig::{
+        TextGenerationAutoUsage, TextGenerationPrefixRequirementType,
+        TextGenerationSenderContextMode,
+    },
     strings,
 };
 
@@ -197,5 +200,43 @@ pub(super) fn determine(
         );
     }
 
+    if let Some(remaining_text) = text.strip_prefix("sender-context-mode") {
+        let remaining_text = remaining_text.trim();
+
+        if !remaining_text.is_empty() {
+            return Err(ControllerType::Error(
+                strings::cfg::configuration_getter_used_with_extra_text(
+                    "sender-context-mode",
+                    remaining_text,
+                )
+                .to_owned(),
+            ));
+        }
+
+        return Ok(ConfigTextGenerationSettingRelatedControllerType::GetSenderContextMode);
+    }
+
+    if let Some(value_string) = text.strip_prefix("set-sender-context-mode") {
+        let value_string = value_string.trim().to_owned();
+        let value_choice = if value_string.is_empty() {
+            None
+        } else {
+            let value_choice =
+                TextGenerationSenderContextMode::from_str(&value_string.to_lowercase());
+
+            if value_choice.is_none() {
+                return Err(ControllerType::Error(
+                    strings::cfg::configuration_value_unrecognized(&value_string).to_owned(),
+                ));
+            }
+
+            value_choice
+        };
+
+        return Ok(
+            ConfigTextGenerationSettingRelatedControllerType::SetSenderContextMode(value_choice),
+        );
+    }
+
     Err(ControllerType::Unknown)
 }