feat: implement sub-agent workflow modes (disabled, sync, async)

[bhouston]

Description

This PR implements GitHub issue #344, adding support for different sub-agent workflow modes.

Changes

• Added a new configuration option subAgentMode with three possible values:
  • disabled: No sub-agent tools are available (default)
  • sync: Parent agent waits for sub-agent completion before continuing (experimental)
  • async: Sub-agents run in the background (experimental)
• Updated the getTools function to conditionally include agent tools based on the configured mode
• Added CLI option support for the new configuration
• Created comprehensive documentation for the new feature in sub-agent-modes.md
• Updated the configuration documentation to include the new option

Documentation

• Added a new documentation page explaining the different sub-agent workflow modes
• Updated the configuration documentation to include the new option
• Provided examples for each mode
• Marked the sync and async modes as experimental in the documentation
• Set 'disabled' as the default mode for better efficiency and safety

Fixes #344