feat(desktop): style structured agent reply labels in the message renderer

[launchmaniac]

Summary

Some agents reply in a fixed, labelled envelope — a status line, an answer block
(broken into "what it is / why it matters / done when"), a source, a confidence,
a next action, an owner, and a blocker/escalation line. This adds a small,
presentation-only rehype plugin (rehypeStructuredReplyLabels) that gives those
labels the existing --primary accent and font-bold weight so the structure
reads at a glance.

It is deliberately narrow and safe:

• Acts only when the message is a valid envelope (the seven top-level labels
  appear line-leading, in the canonical order). Ordinary prose and other
  messages are untouched.
• Styles only the exact labels at line-leading positions; values and body text
  are never styled; it never descends into code/pre/a/blockquote.
• Renderer-only: the stored message text is never altered — labels are wrapped
  for display in the HAST phase, the same place as rehypeSearchHighlight.
• Uses the existing semantic accent token (no hard-coded color) and is wired
  into the single markdown parse choke point (buildMarkdownElement).

Related issue

none found

Testing

• New focused unit tests (rehypeStructuredReplyLabels.test.mjs, 12 cases):
  all ten labels styled; values unstyled; visible text byte-for-byte unchanged;
  false positives rejected (lone/mid-line label, out-of-order, partial envelope,
  label inside inline code, envelope quoted in a blockquote); ordinary Markdown
  (bold/list/link) unchanged.
• Full desktop unit suite pnpm test (4492/4492), pnpm typecheck, and
  biome check all pass.

[launchmaniac]

@block/buzz-oss-team — requesting a review when you have a moment.

Scope: desktop renderer only, three files — a new presentation-only rehype plugin (rehypeStructuredReplyLabels) and its focused test, plus a one-line wiring into the markdown parse path (buildMarkdownElement). Renderer-only: stored message text is never altered.

Local gates (all green): 12/12 focused unit tests, 4492/4492 full desktop unit suite (pnpm test), tsc --noEmit clean, Biome clean.

Visible checks passing: DCO Check and zizmor. CI and Desktop Release Candidate are fork-gated (action_required) — would a maintainer mind approving the workflow runs?

After merge, signed-macos-canary.yml can be run on main to produce the signed artifact.

Thanks!

[launchmaniac]

Withdrawing this submission because the renderer behavior does not meet the expected result. No merge or release was produced.