[FEATURE] Make cross-session peer message collapsing configurable (2.1.247 collapsed them to a one-line preview)

Summary

Since v2.1.247, cross-session peer messages (SendMessage between sessions) are collapsed by default to a one-line preview:

Changed cross-session peer messages to collapse by default to a one-line Message from @<sender>: <first line> preview; Ctrl+O expands the full body

Please make this collapsing configurable — e.g. a peerMessageDisplay: "full" | "collapsed" setting (or a configurable preview line count), so users who coordinate multiple sessions can keep reading these messages inline as before.

Why the current behavior hurts

Peer messages are different in kind from tool output. Tool output is machine-produced material the model consumes — collapsing it is a win. A peer message is conversation content addressed across sessions that the human supervising both sessions needs to read to know what their agents are coordinating. Before 2.1.247 the full body was visible inline; now the user watching two cooperating sessions sees only the first line of each exchange and effectively loses sight of what the sessions are telling each other.

The suggested escape hatch, Ctrl+O, is disproportionate: it toggles the whole transcript view, expanding all tool output in the session. To read one three-line peer message, the user has to wade through everything else that was deliberately collapsed. There is no way to expand just the message.

Workarounds we considered and rejected:

• "verbose": true in settings.json — same overkill as Ctrl+O, permanently.
• Having the receiving agent restate each inbound message in its own visible reply — pays double tokens for the same text, and defeats the purpose of the feature.

Proposal

Any of these would resolve it (in order of preference):

1. A setting, e.g. peerMessageDisplay: "full" | "collapsed" (default can stay collapsed).
2. A configurable preview length (lines), where a large value ≈ full display.
3. A per-message expand affordance (expand this message only, not the whole transcript).

Related signal that display collapsing wants configurability: #91038 asks for collapsible tool output with a settings option; #87574 asks for configurable peer-message label colors. Both point the same direction: these display policies should be settings, not constants.

Environment

• Claude Code v2.1.252 (behavior introduced in 2.1.247 per CHANGELOG)
• macOS (darwin), TUI fullscreen mode
• Reproduce: run two interactive sessions, SendMessage a multi-line body from one to the other — the recipient's terminal renders only Message from @<sender>: <first line>.