feat(swarm): Make entry point configurable

[mkmeral]

Description

Added configurable entry point support to the Swarm multi-agent system. Previously, swarms always started execution with the first agent in the list, which limited flexibility in orchestrating agent workflows. This change introduces an optional entry_point parameter that allows users to specify which agent should initiate the swarm execution.

Key Changes:

• Added entry_point parameter to Swarm constructor (optional, defaults to None)
• Enhanced validation to ensure specified entry points exist in the swarm
• Updated swarm initialization logic to use configured entry point or fall back to first agent
• Maintained full backward compatibility - existing code continues to work unchanged
• Added comprehensive test coverage including validation error scenarios

Usage Example:

# Before: Always starts with first agent
swarm = Swarm([coordinator, analyst, writer])

# After: Can specify which agent starts
swarm = Swarm([coordinator, analyst, writer], entry_point="analyst")

Related Issues

N/A - Feature enhancement request

Documentation PR

N/A - No documentation changes required for this internal API enhancement

Type of Change

New feature

Testing

How have you tested the change?

• Added 3 new unit tests covering:
  • Configurable entry point functionality
  • Invalid entry point validation (including random strings)
  • Default behavior preservation
• Verified all existing swarm tests continue to pass (17/17 passing)
• Confirmed integration tests remain functional (2/2 passing)
• Tested backward compatibility with existing swarm usage patterns

Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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