fix: update deprecated imports and correct documentation

[golldyck]

Problem

The langchain-starter crashes immediately on import because opengradient.mltools no longer exists in the current SDK, and messages_modifier was renamed to state_modifier in langgraph 0.2.60+. The README also claims features (AgentKit, Twitter toolbox) that do not exist in the code.

Fix

Updated imports to use the current opengradient.alphasense module with create_run_model_tool, renamed messages_modifier to state_modifier, and corrected README documentation to match actual functionality.

Changes

• langchain-starter/tools.py — Changed from opengradient.mltools import create_og_model_tool, ToolType to from opengradient.alphasense import create_run_model_tool
• langchain-starter/agent.py — Renamed messages_modifier to state_modifier for langgraph 0.2.60+ compatibility
• README.md — Removed false claims about Coinbase AgentKit and Twitter toolbox integration
• langchain-starter/README.md — Removed references to unused Twitter API keys

Testing

• Verified from opengradient.alphasense import create_run_model_tool resolves correctly
• Confirmed state_modifier is the correct parameter name for the installed langgraph version
• Checked README accurately describes the starter kit contents

Closes #2

[golldyck]

Hi @adambalogh @kylexqian — this PR fixes bugs I found during a full audit of OpenGradient's public repositories. Would appreciate a review when you get a chance. Happy to address any feedback. Thanks!