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>

.github/plugin/marketplace.json

@@ -1339,6 +1339,12 @@
       "description": "Security frameworks, accessibility guidelines, performance optimization, and code quality best practices for building secure, maintainable, and high-performance applications.",
       "version": "1.0.0"
     },
+    {
+      "name": "sentry-triage",
+      "source": "plugins/sentry-triage",
+      "description": "Scan live Sentry issues in a Copilot canvas, group them by urgency, and hand issues off for tracking or a fix PR.",
+      "version": "1.0.0"
+    },
     {
       "name": "signals-dashboard",
       "source": "plugins/signals-dashboard",