fix(acp): keep ordinary DM replies top-level

[funge]

Summary

• instruct agents to send ordinary DM responses in the main timeline
• preserve thread metadata and allow explicitly requested DM thread replies
• retain existing channel threading behavior

Validation

• cargo fmt --check
• cargo test -p buzz-acp (804 unit tests + 9 lifecycle tests passed)
• just ci reached desktop pnpm check, then failed because pnpm dependency fetches returned TypeError: fetch failed; Rust workspace checks completed first

Originating Buzz DM: 44111ea4-ec72-464a-905e-ce65bbb29d4a

[Chessing234]

this is done the right way round — the DM branch stops emitting the anchor rather than just discouraging its use, and both affected tests are renamed and rewritten to assert the new behaviour instead of being deleted. no complaints about the mechanism.

one line i'd question: "If the requested destination is ambiguous, ask before sending." in a human-agent DM that's clearly right. in an agent-to-agent DM there may be nobody to ask, and the turn ends with a question nobody answers — which looks identical to the agent silently doing nothing. is that branch reachable for agent-to-agent turns, and if so is stalling the intended outcome there, or should it default to top-level and proceed?

not blocking — the rest of the instruction is unambiguous enough that ambiguity should be rare.