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/status.rs

@@ -359,6 +359,33 @@ async fn generate_text_generation_section(
         ),
     );
 
+    // Sender Context
+
+    let effective_sender_context = room_config_context.text_generation_sender_context_mode();
+    let room_config_sender_context = room_config_context
+        .room_config
+        .settings
+        .text_generation
+        .sender_context_mode;
+    let global_config_sender_context = room_config_context
+        .global_config
+        .fallback_room_settings
+        .text_generation
+        .sender_context_mode;
+
+    let sender_context_set_where = if room_config_sender_context.is_some() {
+        strings::cfg::status_badge_set_in_room_config()
+    } else if global_config_sender_context.is_some() {
+        strings::cfg::status_badge_set_in_global_config()
+    } else {
+        strings::cfg::status_badge_using_hardcoded_default()
+    };
+
+    message.push_str(&strings::cfg::status_text_generation_entry_sender_context(
+        effective_sender_context,
+        sender_context_set_where,
+    ));
+
     // Prompt override
 
     let text_agent_prompt = if let Some(text_generation_agent) = &text_generation_agent {