fix(acp): keep all direct-message replies flat

[kerpopule]

Summary

Keep managed-agent direct messages as a normal flat conversation. Every ordinary DM response is sent at the channel top level, even when the incoming DM carries legacy thread tags. Human-facing channel replies retain their existing thread-root behavior.

This intentionally applies the flat behavior more broadly than #3077. It resolves the top-level case in #3075 and removes the compounding nested-DM behavior described in #2748 by eliminating DM reply anchors rather than moving them to a root.

Implementation

• Add a DM-specific prompt instruction that forbids --reply-to for both top-level and previously threaded DM events.
• Remove DM reply-anchor generation while leaving non-DM anchor resolution unchanged.
• Align the shared base prompt with the flat-DM contract.
• Cover top-level DMs, legacy threaded DMs, and the shared prompt contract with regression tests.

Verification

• cargo fmt --all -- --check
• env -u BUZZ_ACP_LAZY_POOL cargo test -p buzz-acp: 662 unit tests and 9 integration tests passed
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• Existing channel-thread anchoring tests pass

just ci reached the mobile suite, where the unrelated ChannelDetailPage keeps follow mode off while a tall newest message stays visible test failed. This change does not touch mobile code.

Fixes #3075.
Addresses #2748 with the product behavior requested here: no threads in direct messages.