fix(acp): keep top-level agent replies in channel

[inde3d100]

What changed

• keep agent replies to top-level channel messages in the channel timeline
• continue flattening human-facing replies made inside an existing thread to that thread's root
• apply the same channel-versus-thread behavior to setup-mode nudges
• update ACP prompt guidance and reply-routing tests for top-level, threaded, batched, human, and agent-to-agent cases

Why

The ACP harness currently instructs an agent to reply to every top-level human mention with --reply-to, which automatically turns that exchange into a thread. This makes normal channel conversation disappear behind threads even when the human did not start one.

The desired interaction is:

• top-level message → top-level reply in the same channel
• message already in a thread → reply at that thread root

This keeps the channel readable while preserving deliberate threaded conversations.

Validation

• just ci
• all 649 buzz-acp tests passed
• desktop, web, and mobile cross-platform checks passed
• git diff --check origin/main...HEAD