#25271 fix(next): agent reports start as one compact line

Received agent messages start as one line: sender, ellipsized preview and time. Clicking the row opens the complete selectable message; nested actions and text selections keep their existing behavior. Sent messages get the same whole-row disclosure target. The row estimate matches each state so opening a card keeps the existing anchored-collapse behavior.

Leo (2026-10-26): “the sort of single-line-ish type thing would be pretty good, like received message ... would we be able to, like, just have a symbol for the received message from or whatever?”

Global Minimal Mode is currently off by default. This change makes agent messages compact independently of that setting. Consecutive-message grouping is a separate follow-up: its cache needs to preserve change boundaries through production overlays before it meets the no-new-O(rows)-per-token requirement. This PR adds no row projection, timer or transcript-wide token work.

Gallery: agent-pane.incoming-message/messages, agent-pane.incoming-message/20-compact, agent-pane.incoming-message/20-expanded, agent-pane.incoming-message/20-minimal

Design reference: cmux-design-references apps/claude/live/transcript-collapsed-agent-message.png and transcript-received-messages-group.png. The existing anchored collapse is reused; identical messages do not animate again.

Observed / Not run

Hosted run 40259515424, exact source f50bed1f3c1dcfa6065ba229c0f0e967b516f569: the compact-default regression fails on its test-only baseline, and the selection regression fails before its fix; all 45 focused tests, including real Chromium disclosure/keyboard tests, then pass. Full new-only webview checks against e9775bebab98 pass. No native build was needed or run.

Before/after gallery captures and an independent read-only Codex gpt-6-astra xhigh review are in progress; nx-chat owns that final loop and landing.

Agent session: big-red

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