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/agent/provider/openai/controller.rs

@@ -67,6 +67,7 @@ impl ControllerTrait for Controller {
 
         let messages = vec![LLMMessage {
             author: LLMAuthor::User,
+            sender_id: None,
             content: LLMMessageContent::Text("Hello!".to_string()),
             timestamp: chrono::Utc::now(),
         }];
@@ -104,6 +105,7 @@ impl ControllerTrait for Controller {
         } else {
             Some(LLMMessage {
                 author: LLMAuthor::Prompt,
+                sender_id: None,
                 content: LLMMessageContent::Text(prompt_text),
                 timestamp: chrono::Utc::now(),
             })