Add create_triage_agent helper function

[alpnix]

Summary

Implements the missing create_triage_agent function that is referenced in the personal_shopper example but was never implemented in the codebase.

Changes

• Added new swarm/agents.py module with create_triage_agent helper function
• Updated swarm/__init__.py to export the new function
• Function dynamically generates transfer functions for routing conversations to sub-agents
• Supports optional backlinks for returning to the triage agent

Implementation Details

• Follows existing code style and conventions from core.py and util.py
• Uses proper type hints and comprehensive docstrings
• Handles agent name to function name conversion (spaces to underscores, lowercase)
• Avoids mutating original agent function lists when adding backlinks

Testing

• Verified syntax with py_compile
• Tested logic with standalone unit tests
• Follows the pattern used in the airline example but provides a cleaner API

Fixes

Closes #57
Closes #49

Example Usage

from swarm import Agent
from swarm.agents import create_triage_agent

sales_agent = Agent(name="Sales Agent", instructions="Handle sales")
refunds_agent = Agent(name="Refunds Agent", instructions="Handle refunds")

triage = create_triage_agent(
    name="Triage Agent",
    instructions="Route users to the right agent",
    agents=[sales_agent, refunds_agent],
    add_backlinks=True
)