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/tests.rs

@@ -163,6 +163,26 @@ fn determine_controller() {
                 ),
             )),
         },
+        TestCase {
+            name: "per-room text-generation/sender-context-mode getter",
+            input: "room text-generation sender-context-mode",
+            expected: super::ControllerType::Config(controller_type::ConfigControllerType::SettingsRelated(
+                controller_type::SettingsStorageSource::Room,
+                controller_type::ConfigSettingRelatedControllerType::TextGeneration(
+                    controller_type::ConfigTextGenerationSettingRelatedControllerType::GetSenderContextMode,
+                ),
+            )),
+        },
+        TestCase {
+            name: "global text-generation/sender-context-mode getter",
+            input: "global text-generation sender-context-mode",
+            expected: super::ControllerType::Config(controller_type::ConfigControllerType::SettingsRelated(
+                controller_type::SettingsStorageSource::Global,
+                controller_type::ConfigSettingRelatedControllerType::TextGeneration(
+                    controller_type::ConfigTextGenerationSettingRelatedControllerType::GetSenderContextMode,
+                ),
+            )),
+        },
         TestCase {
             name: "per-room text-to-speech/speed-override getter",
             input: "room text-to-speech speed-override",