Adds agent_responds_in_dm_e2e — completes the functional trio (public
channel, private channel, DM). A DM in serverless is a private channel
with a deterministic UUIDv5 id derived from the participants (matching
desktop/src-tauri/src/commands/dms.rs). The message is gift-wrapped to
the agent; the test asserts the agent decrypts it and replies.
All three pass against live public relays (3 passed; 0 failed).
a4183e263c