feat(desktop): surface buried agent-to-human replies to channel timeline

[wpfleger96]

Summary

Buried agent-to-human replies from nested sub-threads now surface to the channel timeline as read-time pointer rows, inserted at the reply's own createdAt — where the reader's eye actually is. The real message is never moved or copied; clicking the pointer navigates down to the still-nested original and highlights it.

This is the read-side complement to the reply-routing tree: it makes mis-routed (buried) agent-to-human replies visible without altering routing or the underlying event graph.

Design

• Pointer rows, not moves. surfaceReplies is a pure projection that identifies agent-to-human replies nested below the root and emits a synthetic pointer at each reply's own createdAt. The original message stays nested.
• Merge by sort, not append. mergeSurfacedReplies concatenates pointers with the main timeline and sorts by (createdAt, id) on the real message id, so a pointer lands chronologically where the human is reading.
• Separate component. SurfacedReplyRow reuses the thread-summary pill visual (rounded button, avatar, muted "replied" label) without forcing synthetic summary semantics through MessageThreadSummaryRow.
• Navigate down. Clicking routes via goChannel(channelId, { messageId, threadRootId }) — opens the thread at its root, scrolls to and highlights the specific nested reply. threadRootId gates the root-event prefetch in ChannelRouteScreen so the open never dead-clicks when the thread root sits outside the loaded window. This is the same path search hits use.

Classification fail-safe

The human/agent classifier resolves unknown pubkeys to human (pubkey == null || !agentPubkeys?.has(pubkey)), so an unrecognized author under-surfaces rather than mis-surfaces — honoring the pure-core's documented contract end to end.

Quality

Both phases certified 9/9/9 on minimalism, elegance, and correctness across a two-pass review:

• Phase 1 (pure core): cross-thread false-dedupe fixed via composite threadKey; empty-body and malformed-tag edge cases pinned; isRootLevel drift contract locked with a broadcast-branch test.
• Phase 2 (render wiring): isHuman fail-safe restored end to end; threadRootId prefetch confirmed load-bearing for the distant-root navigate-down case.

just desktop-typecheck exit 0, just desktop-check clean (2 pre-existing unowned getMockProfile warnings), just desktop-test 734/734.

[wpfleger96]

Surfaced reply appears in timeline

Agent (alice) replies nested to a human's root message, mentioning the user — a pointer row surfaces in the channel timeline at the reply's own timestamp.

Multiple surfaced replies interleave chronologically

Two threads, each with an agent reply mentioning a human — both surface as pointer rows, sorted by createdAt among root messages.

Agent-to-agent reply does NOT surface

A nested agent reply that only mentions another agent — no pointer row appears. The surfacing filter correctly requires a human p-tag.

Click navigates to nested message

Clicking a surfaced-reply pointer opens the thread panel and highlights the actual nested reply in context.