feat(05-04): implement @mention message handler

Add mention_handler for @claude, @gpt, @gemini direct messages:
- Parse model name from @mention prefix
- Get active discussion for context (if exists)
- Query model via query_model_direct with full context
- Persist response with is_direct=True flag
- Register MessageHandler with regex filter after CommandHandlers

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/moai/bot/handlers/__init__.py

@@ -4,12 +4,13 @@ This module contains handlers for Telegram bot commands including
 project management, discussion commands, and export functionality.
 """
 
-from telegram.ext import Application, CommandHandler
+from telegram.ext import Application, CommandHandler, MessageHandler, filters
 
 from moai.bot.handlers.commands import help_command, start_command
 from moai.bot.handlers.discussion import (
     ask_command,
     discuss_command,
+    mention_handler,
     next_command,
     open_command,
     stop_command,
@@ -41,3 +42,7 @@ def register_handlers(app: Application) -> None:
     app.add_handler(CommandHandler("discuss", discuss_command))
     app.add_handler(CommandHandler("next", next_command))
     app.add_handler(CommandHandler("stop", stop_command))
+
+    # @mention handler - MessageHandler registered AFTER CommandHandlers
+    # Matches messages starting with @claude, @gpt, or @gemini followed by content
+    app.add_handler(MessageHandler(filters.Regex(r"^@(claude|gpt|gemini)\s"), mention_handler))

src/moai/bot/handlers/discussion.py

@@ -6,13 +6,14 @@ from telegram.ext import ContextTypes
 from moai.bot.handlers.projects import get_selected_project
 from moai.core.ai_client import MODEL_MAP, get_ai_client
 from moai.core.models import DiscussionType, RoundType
-from moai.core.orchestrator import query_models_parallel, run_discussion_round
+from moai.core.orchestrator import query_model_direct, query_models_parallel, run_discussion_round
 from moai.core.services.discussion import (
     complete_discussion,
     create_discussion,
     create_message,
     create_round,
     get_active_discussion,
+    get_current_round,
     get_discussion,
 )
 
@@ -343,3 +344,84 @@ async def stop_command(update: Update, context: ContextTypes.DEFAULT_TYPE) -> No
 
     except Exception as e:
         await update.message.reply_text(f"Error: {e}")
+
+
+async def mention_handler(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Handle @model mention messages.
+
+    Messages starting with @claude, @gpt, or @gemini are routed to that
+    specific model. If a discussion is active, includes full context.
+
+    Examples:
+        @claude What do you think about this?
+        @gpt Can you elaborate on your previous point?
+        @gemini Do you agree with Claude?
+    """
+    message_text = update.message.text
+    if not message_text:
+        return
+
+    # Parse model name from first word (e.g., "@claude" -> "claude")
+    parts = message_text.split(maxsplit=1)
+    if not parts:
+        return
+
+    model_tag = parts[0].lower()
+    # Strip the @ prefix
+    model_name = model_tag.lstrip("@")
+
+    # Validate model
+    if model_name not in MODEL_MAP:
+        return  # Not a valid model mention, ignore
+
+    # Get the rest of the message as content
+    content = parts[1] if len(parts) > 1 else ""
+    if not content.strip():
+        await update.message.reply_text(
+            f"Usage: @{model_name} <message>\n\nExample: @{model_name} What do you think?"
+        )
+        return
+
+    # Get project context if available
+    project = await get_selected_project(context)
+    if project is None:
+        await update.message.reply_text("No project selected. Use /project select <name> first.")
+        return
+
+    project_name = project.name
+
+    # Check for active discussion (for context)
+    discussion = await get_active_discussion(project.id)
+
+    # Show typing indicator
+    await update.message.chat.send_action("typing")
+
+    try:
+        # Query the model directly with optional discussion context
+        response = await query_model_direct(
+            model=model_name,
+            message=content,
+            discussion=discussion,
+            project_name=project_name,
+        )
+
+        # If there's an active discussion, persist the message
+        if discussion is not None:
+            # Get or create a round for this direct message
+            current_round = await get_current_round(discussion.id)
+            if current_round is not None:
+                await create_message(
+                    round_id=current_round.id,
+                    model=model_name,
+                    content=response,
+                    is_direct=True,
+                )
+
+        # Format response
+        await update.message.reply_text(
+            f"*@{model_name.title()} (direct):*\n{response}",
+            parse_mode="Markdown",
+        )
+
+    except Exception as e:
+        await update.message.reply_text(f"Error: {e}")