fix(ui): single underline on markdown document card labels (#18215)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Comments and task bodies render markdown, and a saved plan or
document shows as a compact document card
> - The document card is a link inside the markdown body, so it gets the
general markdown link underline
> - The card label also has its own underline, so the label shows two
lines
> - This pull request removes the inherited underline from document
cards only
> - The benefit is that the card label shows one clean underline

## Linked Issues or Issue Description

No public issue exists. This is a small visual bug.

**What happened?**

A document card (for example the "plan" card) in a comment shows a
double underline under its label.

**Expected behavior**

The card label shows a single underline.

**Steps to reproduce**

1. Open a task that has a plan document.
2. Look at a comment or body that links the plan as a document card.
3. See two underlines under the card label.

**Paperclip version or commit**

`master` at 649354d8c.

**Deployment mode**

Local development (`pnpm dev`). The bug is in shared UI CSS, so it
affects all modes.

Related PRs: I searched for "document card underline" and found no
duplicate PRs.

## What Changed

- `ui/src/index.css`: add `.paperclip-markdown
a[data-sidebar-document-link] { text-decoration: none; }` so the card
does not inherit the markdown link underline. The label span keeps its
own underline.
- `ui/src/components/MarkdownDocumentCardStyles.test.ts`: add a
stylesheet regression test. It uses the same pattern as
`MarkdownAccentStyles.test.ts`.

## Verification

- `pnpm --filter @paperclipai/ui exec vitest run
src/components/MarkdownDocumentCardStyles.test.ts
src/components/MarkdownExternalLink.test.tsx` passes (11 tests).
- `node scripts/check-token-gates.mjs` reports all gates clean.
- Manual: open a comment with a document card. The label shows one
underline. Other markdown links keep their underline.
- Snapshot baselines are intentionally not updated. See the
`doc/design/DECISION-SHEET.md` entry "Per-change snapshot verification
demoted to dormant (Jul 13 2026)".

## Risks

- Low risk. The selector targets only anchors with
`data-sidebar-document-link` inside `.paperclip-markdown`. Other links
do not change.

## Model Used

- Anthropic Claude Opus 5.5 (`claude-opus-5-5`), used through Claude
Code with tool use (shell, file edit). Standard context window. No
extended thinking mode.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

ui/src/components/MarkdownDocumentCardStyles.test.ts

@@ -0,0 +1,24 @@
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { describe, expect, it } from "vitest";
+
+const stylesheet = readFileSync(fileURLToPath(new URL("../index.css", import.meta.url)), "utf8");
+
+function cssBlock(selector: string): string {
+  const start = stylesheet.indexOf(`${selector} {`);
+  expect(start, `Missing CSS selector: ${selector}`).toBeGreaterThanOrEqual(0);
+
+  const bodyStart = stylesheet.indexOf("{", start);
+  const bodyEnd = stylesheet.indexOf("\n}", bodyStart);
+  expect(bodyStart, `Missing CSS block start: ${selector}`).toBeGreaterThanOrEqual(0);
+  expect(bodyEnd, `Missing CSS block end: ${selector}`).toBeGreaterThan(bodyStart);
+
+  return stylesheet.slice(bodyStart + 1, bodyEnd);
+}
+
+describe("markdown document card styles", () => {
+  it("removes the inherited markdown link underline so the card label shows one underline", () => {
+    expect(cssBlock(".paperclip-markdown a")).toContain("text-decoration: underline");
+    expect(cssBlock(".paperclip-markdown a[data-sidebar-document-link]")).toContain("text-decoration: none");
+  });
+});

ui/src/index.css

@@ -2549,6 +2549,12 @@ a.paperclip-mention-chip[data-mention-kind="agent"]::before {
   text-decoration: none;
 }
 
+/* Document cards underline only their label span; the card itself must not
+   inherit the markdown link underline, or the label shows two lines. */
+.paperclip-markdown a[data-sidebar-document-link] {
+  text-decoration: none;
+}
+
 .paperclip-markdown a.paperclip-mention-chip[data-mention-kind="issue"] {
   border-color: color-mix(in oklab, var(--foreground) 14%, var(--border) 86%);
   background: color-mix(in oklab, var(--accent) 42%, transparent);