feat(newtab): Add hnt-backend-investigation skill

plugins/newtab/README.md

@@ -9,3 +9,21 @@ Skills and tools for Firefox newtab development.
 Adds `@nova-cleanup` comments to Firefox newtab code for Project Nova parallel implementation tracking.
 
 Use this skill when adding cleanup comments to Nova-related changes. See [`skills/nova-cleanup-comments/SKILL.md`](skills/nova-cleanup-comments/SKILL.md) for details.
+
+### `hnt-backend-investigation`
+
+Diagnostic workflow for Home New Tab **backend** incidents, covering Merino, the curated corpus, admin-api, the article crawler, and the ML section pipeline. Its output is an evidence-backed `FINDINGS.md` carrying a root cause and quantified impact.
+
+Use this skill when a Sentry alert fires, an editor reports something broken, or recommendations look empty, wrong, or stale. See [`skills/hnt-backend-investigation/SKILL.md`](skills/hnt-backend-investigation/SKILL.md) for details.
+
+**Optional tools:**
+
+- `gh`
+- `gcloud` and `bq`
+- the `aws` CLI, with a read-only SSO profile
+- a MySQL client, plus Mozilla VPN
+- `uv` or `python3`
+- the Sentry MCP server
+- the Slack MCP server, for `#hnt-dev-be-alerts`
+
+Anything unreachable is raised as a task and reported under "Could not measure".