Issue 289 supervisor v2

[tikhandesanket]

Summary

This PR revises the Dynaflow multi-agent supervisor implementation for issue #289.

Changes

• Add named specialist routing through the supervisor.
• Add explicit handoff requests and a supervisor method to invoke the requested specialist with shared workflow state.
• Preserve the existing Handoff(state) passthrough behavior.
• Pass shared state into synchronous and asynchronous ReAct execution.
• Add tests for specialist routing, handoff behavior, shared state, and aggregation.

Validation

• Focused tests and the full test suite passed locally.
• Ruff checks for the changed files passed.
• Pre-commit hooks passed.

Scope note

Handoff requests are executed explicitly through Supervisor.handoff(). The graph-routing tests separately validate conditional routing and state preservation; automatic graph consumption of handoff requests is not introduced by this PR.

Closes #289.

[tikhandesanket]

@codeforstartups @Kaap10 please have a look