Add Sentry Triage canvas extension (#2802)

An on-call error-triage canvas for GitHub Copilot: scans live Sentry
issues, groups them by urgency, and hands issues off for tracking or a
fix PR. Registers a reusable canvas extension under extensions/ with a
matching plugins/sentry-triage manifest.

The extension runs a loopback HTTP/SSE server per canvas instance
(host-allow-listed, per-instance capability token never broadcast over
SSE), renders the triage board server- and client-side, and drives
model turns over a single shared Copilot session serialized so scan and
work turns can't interleave. Work state is prototype-pollution-guarded
against agent-controlled tracking keys, and sendAndWait timeouts are
handled without duplicating external side effects.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

extensions/sentry-triage/components/category.mjs

@@ -0,0 +1,29 @@
+import { Card } from './card.mjs'
+import { escapeHtml } from '../escape.mjs'
+
+const CATEGORY_DESCRIPTIONS = {
+  'regressions': 'Issues that previously existed, were resolved, and have resurfaced — likely tied to a recent release.',
+  'escalating': 'Issues Sentry flagged as escalating, plus older high-impact issues affecting many users or generating many events.',
+  'new-critical': 'Brand new issues that have already hit multiple users within hours of first appearing.'
+}
+
+export function Category({ id, name, issues, plainEnglishView, availableModels = [] }) {
+  if (!issues.length) return ''
+
+  const description = CATEGORY_DESCRIPTIONS[id] || ''
+  const safeId = escapeHtml(id)
+
+  return `<section class="category" data-category="${safeId}">
+    <div class="category-header">
+      <h2>${escapeHtml(name)} <span class="badge">${issues.length}</span></h2>
+      <label class="category-select-all">
+        <input type="checkbox" class="category-check" data-category="${safeId}" />
+        Select all
+      </label>
+      ${description ? `<p class="category-description">${description}</p>` : ''}
+    </div>
+    <div class="card-list">
+      ${issues.map((issue) => Card({ ...issue, plainEnglishView, availableModels })).join('')}
+    </div>
+  </section>`
+}