[Idea] Fold surface-replaced history in the Web transcript (compaction and plugin replace markers)

[William20100221]

Motivation

When a surface replace lands, the history it shadowed stays fully expanded in the Web/Desktop transcript. For compaction this is by design: CompactionSummaryNode says the shadowed conversation "stays in the transcript above it" (packages/client/ui-conversation/src/client/contract/records.ts:186-191). In long sessions that means scrolling through many messages the model can no longer see, with no visual cue for where the model's context actually starts.

The same mechanism is open to plugins: SurfaceOp documents that "any surface-replacing producer may use it" (packages/core/session/src/types.ts:467). My plugin dsh-clear-conversation uses it so a user can say "clear everything above, then …" or type /clear: earlier turns are replaced by a short marker and stop being sent. Users then asked for the cleared messages to fold away, like the reasoning block, instead of staying expanded above the clear.

A plugin cannot do this today:

• Conversation node claims are decided per event at dispatch (ui-conversation/.../conversation/assembler.ts, exclusive handling around lines 522-542), so a later replace event cannot reclaim rows other definitions already built.
• The chat target allows one ConversationGroupDefinition (conversation/group-registry.ts:24-38 throws "already registered"), and ui-chat already registers processGroupDefinition (ui-chat/src/client/conversation-nodes/register.ts:37).

The data is already on the wire: replacement events carry surfaceOp and sourceEventSeqs (packages/api/session-controller/src/types.ts:462-464).

Behavior

Proposed, Web and Desktop chat only; the session log and the model-visible surface do not change:

1. When a replacement event's sourceEventSeqs shadow earlier transcript rows, the chat folds those rows into one collapsible block at the replacement point, collapsed by default, reusing the disclosure style of the reasoning row.
2. The block header says how many messages left the model's context, for example "12 earlier messages are no longer in the model's context". Expanding shows the original rows unchanged.
3. It applies to compaction checkpoints and to any producer's replace marker. A producer can supply its own header, for example through a keyed slot such as conversation.chat.fold.header keyed by the marker's source.kind, with the default header as fallback.
4. A user setting can turn folding off for people who prefer today's fully expanded transcript.

A smaller alternative that would also unblock plugins: an API for a client plugin to mark transcript rows as folded under a node it owns, or allowing more than one group definition per target.

Related but separate: #6850 asks for compaction hooks that change what the model sees; this idea only changes what the user sees.

Drafted with AI assistance and checked against the 0.2.1-rc.5 sources.