fix: add youtube_transcript and transcribe_audio to TOOL_FUNCTIONS

Bug: Tools were registered in TOOLS dict but missing from TOOL_FUNCTIONS
mapping in graph.py, causing "Tool 'youtube_transcript' not found in TOOL_FUNCTIONS" error.

Fixed:
- Added imports: youtube_transcript, transcribe_audio
- Updated TOOL_FUNCTIONS dict in execute_node (line 268)
- Updated TOOL_FUNCTIONS dict in fallback section (line 410)

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

src/agent/graph.py

@@ -19,7 +19,7 @@ from pathlib import Path
 from typing import TypedDict, List, Optional
 from langgraph.graph import StateGraph, END
 from src.config import Settings
-from src.tools import TOOLS, search, parse_file, safe_eval, analyze_image
+from src.tools import TOOLS, search, parse_file, safe_eval, analyze_image, youtube_transcript, transcribe_audio
 from src.agent.llm_client import (
     plan_question,
     select_tools_with_function_calling,
@@ -270,6 +270,8 @@ def execute_node(state: AgentState) -> AgentState:
         "parse_file": parse_file,
         "calculator": safe_eval,
         "vision": analyze_image,
+        "youtube_transcript": youtube_transcript,
+        "transcribe_audio": transcribe_audio,
     }
 
     # Initialize results lists
@@ -410,6 +412,8 @@ def execute_node(state: AgentState) -> AgentState:
                     "parse_file": parse_file,
                     "calculator": safe_eval,
                     "vision": analyze_image,
+                    "youtube_transcript": youtube_transcript,
+                    "transcribe_audio": transcribe_audio,
                 }
 
                 for tool_call in tool_calls: