Implement CE-WP-0004 T01-T05: citation card export (Markdown + HTML)

Per-evidence-item export: click Export → Copy as Markdown / Copy as HTML
writes a portable citation card to the clipboard. Cmd/Ctrl+Shift+C
exports the active evidence as Markdown.

- ADR-0007 locks the Markdown + HTML output formats.
- New shared types: CitationCard, openContextUrl(), resolveSourceLabel().
- Engine renderers under src/engine/rendering/: renderCitationCardMarkdown,
  renderCitationCardHtml — snapshot-tested, escape-safe, BEM classes for HTML.
- src/work/useExportEvidence.ts wires engine + renderers + clipboard.
- EvidenceSidebar gains an Export popover per row + auto-dismissing toast.
- E2E test (tests/integration/citation-card-export-e2e.dom.test.tsx)
  walks PRD scenario steps 10-11 and asserts the clipboard payload.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/decisions/ADR-0007-citation-card-format.md

@@ -0,0 +1,115 @@
+# ADR-0007 — Citation card output format (Markdown and HTML)
+
+- Status: accepted
+- Date: 2026-05-25
+- Workplan: CE-WP-0004-T02 (Markdown renderer) and CE-WP-0004-T03 (HTML renderer)
+- Spec refs: `wiki/ArchitectureOverview.md` §4.7, §14.1, §14.2, §14.3
+
+## Context
+
+The MVP scenario ends with a user exporting an evidence item as a portable
+citation card. Two formats ship in CE-WP-0004:
+
+- **Markdown** — copied to the clipboard for pasting into notes, emails,
+  GitHub issues, and so on. Renders well as plain text and as rendered
+  Markdown.
+- **HTML** — copied for pasting into rich-text editors and web pages.
+
+A third format, the `<citation-card>` Web Component from
+`ArchitectureOverview.md` §14.2, is out of scope here and lands in a later
+workplan. Its visual presentation should be *equivalent* to the HTML form
+but is not constrained to be byte-identical.
+
+The two formats need a written contract so that:
+
+1. UI components (T04 sidebar export, future web embeds) can rely on the
+   exact output structure.
+2. Snapshot tests fail loudly if the format drifts.
+3. Consumers that style the HTML form know which elements and classes are
+   stable.
+
+## Decision
+
+### Markdown format (CE-WP-0004-T02)
+
+```markdown
+> {quote}
+
+— *{sourceLabel}* · [Open source]({openContextUrl})
+
+{commentary}
+```
+
+Rules:
+
+- Each `{quote}` line is rendered with the leading `> ` blockquote marker,
+  preserving line breaks in the source quote. A single-line quote is one
+  blockquote line; a multi-line quote becomes multiple `> `-prefixed
+  lines.
+- A blank line follows the blockquote.
+- The attribution line uses an em dash (`—`, U+2014) followed by a single
+  space, the italicised source label, a middle dot (`·`, U+00B7) with
+  surrounding spaces, and the `[Open source]({openContextUrl})` link.
+- The middle dot + link segment is **omitted entirely** when no
+  `openContextUrl` is provided (which is unusual but possible for
+  evidence items without an annotation).
+- A blank line follows the attribution.
+- The optional `{commentary}` paragraph is rendered as-is. When absent
+  the trailing blank line and commentary paragraph are both omitted.
+- The output ends with a single trailing newline.
+
+Reserved Markdown characters inside the quote are not escaped — the
+intent is to reproduce the source text verbatim. The blockquote prefix
+already neutralises the most dangerous reflow problems. The
+`{sourceLabel}` is escaped to defuse `*`/`_` only; the link target is
+URL-encoded by `openContextUrl()`.
+
+### HTML format (CE-WP-0004-T003)
+
+A single `<aside class="citation-card">` root element with this stable
+structure:
+
+```html
+<aside class="citation-card">
+  <blockquote class="citation-card__quote">{escaped quote}</blockquote>
+  <p class="citation-card__attribution">
+    <cite class="citation-card__source">{escaped source label}</cite>
+    <a class="citation-card__link" href="{open context url}">Open source</a>
+  </p>
+  <div class="citation-card__commentary">{escaped commentary}</div>
+</aside>
+```
+
+Rules:
+
+- All user-supplied text is HTML-escaped (`&`, `<`, `>`, `"`, `'`).
+- Inline styles are **not** emitted. Host pages provide the CSS.
+- The `<a>` and the attribution `·` separator are omitted when no
+  `openContextUrl` is provided.
+- The `<div class="citation-card__commentary">` is omitted when no
+  commentary is provided.
+- Commentary is treated as plain text — no Markdown or raw HTML
+  passthrough. A future workplan can introduce a sanitiser if rich
+  commentary is required.
+- The output ends with a single trailing newline.
+
+### Class-name contract
+
+The four BEM-style class names — `citation-card`, `citation-card__quote`,
+`citation-card__attribution`, `citation-card__source`,
+`citation-card__link`, `citation-card__commentary` — are part of the
+public contract. They must not be renamed without an ADR superseding
+this one.
+
+## Consequences
+
+- Snapshot tests in `src/engine/rendering/*.test.ts` lock these
+  formats. Intentional changes require updating both the snapshots and
+  this ADR.
+- The Web Component planned for §14.2 will reuse the HTML structure
+  inside its shadow DOM, so the class names also become the
+  customisation surface for downstream stylesheets.
+- The `openContextUrl` shape from
+  `wiki/ArchitectureOverview.md` §14.3 is now consumed by two
+  renderers; changing the URL scheme requires regenerating snapshots
+  and announcing via a new ADR.