feat: render assistant answers through a sanitized markdown allowlist

[caitlon]

Problem

An assistant answer can carry text the model copied from project data, such as a project name or an expert's position, so whatever renders it must not turn that text into an image, a link to another address, or raw HTML.

Change

Adds react-markdown pinned at 10.1.0 and AssistantMarkdown, which renders an answer from the elements p, strong, em, ul, ol, li, code and br only. unwrapDisallowed keeps the text of anything else, so a markdown link, an autolink or a heading shows as plain text and an image renders nothing; skipHtml drops raw HTML. A [n] marker in the text of a paragraph, list item, bold or italic run stays plain text while the caller passes no source list (an answer still streaming has none yet); once a list is passed, the marker becomes an in-page anchor #assistant-src-<messageId>-<n> when the answer has source n, and its click calls onCite(n) instead of changing the location; a marker with no such source is struck through and is not a link, and its digits are shown as written. Markers inside inline code stay literal. The overrides are module-level components that read the citation data from a context, so a re-render or a streamed token does not remount the answer and a clicked citation keeps focus. isAllowedAssistantUrl accepts only a string that starts with https:// and parses as an https: URL; the source list in the next PR uses it. Nothing renders the component yet: the panel wires it in later.

Verification

New tests in AssistantMarkdown.test.tsx and url-allowlist.test.ts cover the allowed formatting, links, autolinks and headings coming out as text, javascript: links, hard line breaks, images (no element and no alt text), raw HTML, no leaked node attribute, a [n] with no source list, a known and an unknown [n] (including a number too large for a safe integer), a click that is default-prevented and leaves location.hash unchanged, citations inside nested emphasis and inside code, the anchor and paragraph DOM nodes surviving a re-render with a new source list and handler, and the paragraph node surviving appended content. npm run lint, npm run typecheck, npm run test:coverage and ./scripts/ci/ci-local.sh fast are green.

Migrations

None.

Screenshots

None, nothing renders the component yet.

[greptile-apps]

Automatic reviews are disabled for this repository.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud