#28018 fix(acpmux): keep transcript disclosure carets inline

Austin, relayed by Leo: “I think this expansion caret is too much to the right?”
Leo: “1 is ours 2 is claude code” / “we can fit stuff closer”

Changelog

• Fixed: transcript disclosure carets sit immediately after the label using the shared spacing token. Applies to harness headers, tool/command groups, commands, thoughts, and agent-message previews.
• Durations/statuses and message actions retain their trailing position. The message toggle still fills the available row so blank-space clicks expand it. Nested steps retain their existing grid and glyph sizing.

Verification

• Updated the old trailing-column assertions deliberately; the browser regression covers inline gaps in both themes and at narrow/wide widths, nested steps, and the message's blank-space click target.
• Exact-head fleet tests and web checks: run, pending. Earlier dispatches were cancelled while queued before any test step; they provide no validation result.
• Independent Codex review requested; the previous blocking findings are being repaired and require a fresh exact-head verdict.
• The Linux gallery fixture saves 2x light/dark captures with synthetic content. Private reference screenshots are not attached.

Design: chat transcript; this request supersedes the former far-right disclosure column only. Risk is CSS row geometry; existing content, colours, glyphs, actions, and the subagent tray are unchanged.

Agent-Session: https://cmux.com/deeplink/agent?id=aid-efd8e98f8bf683675384