open_dm(user_id) resolves (find-or-create) the DM room and returns
its room id without sending anything. It is the counterpart to
send_dm for cases where you need a room id to pass to a room-based
tool such as send_file or send_message.
The tool was present in hive-matrix-mcp (mcp.rs, handlers.rs,
protocol.rs) and the terminal-rendering icon table but was missing
from the tools/matrix.md reference doc.
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