fix(acp): reply to top-level DMs

[ScaleLeanChris]

Summary

• Give every DM turn the triggering event as its reply anchor.
• Use the existing new-thread send instruction for top-level DMs.
• Correct the existing queue test that encoded the missing-instruction behavior.

Root cause

format_prompt did not produce a reply anchor for a top-level DM, and format_context_hints only emitted a send instruction inside the threaded-DM branch. The harness could therefore complete the ACP turn without calling buzz messages send.

Top-level DMs should no longer end in the sound of silence.

Verification

• cargo test -p buzz-acp test_reply_instruction_present_for_top_level_dm
• cargo test -p buzz-acp queue::tests (138 passed)
• cargo test -p buzz-acp (830 unit tests and 9 lifecycle tests passed)
• cargo clippy -p buzz-acp --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• just ci (passed, including 1,874 mobile tests)

Live relay verification was not run against this source build. The regression test exercises the prompt sent to the harness and asserts the exact reply anchor and new-thread instruction.

Closes #6984

[github-actions]

🔐 Codex Security Review

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