feat: add send_proactive_message and send_proactive_dm

[syasini]

Summary

• Adds send_proactive_message(channel, text, thread_ts=None) to SlackBot — sends a Slack message and pre-populates the corresponding LangGraph thread with it as an AIMessage, so the agent has full context when the user replies in-thread
• Adds send_proactive_dm(user_id, text) as a convenience wrapper that opens the DM channel from a user ID automatically
• Adds plant_agent_langgraph.py example — pure LangGraph (StateGraph + ToolNode) variant of the plant bot, useful for verifying streaming plan/task card behavior with AIMessageChunk.tool_call_chunks

Test plan

• Call send_proactive_dm("U...") from an external scheduler/trigger and verify the message appears in the user's DM
• Reply to the proactive message in-thread and verify the agent has context (sees the proactive message as prior AIMessage in history)
• Call send_proactive_message(channel, text, thread_ts=...) to reply into an existing thread and verify thread ID continuity
• Verify that a top-level DM reply (not in-thread) starts a fresh LangGraph thread as expected

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