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

[mmiermans]

Adds hnt-backend-investigation to the newtab plugin: a diagnostic workflow for Home New Tab backend incidents across 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. Two reference files hold the detail: references/data-sources.md for how to reach each system plus the data traps that silently produce wrong answers, and references/failure-modes.md for failure classes, invariants to check, and falsification moves. Table shapes, GCP project ids, the Merino corpus endpoint, and the Zyte API parameters were each verified against the live systems and the service repos. Docs/skill only; no code changes.

Testing steps:

• Install this branch and run Claude Code from any directory
• Invoke the skill with something like "newtab DE sections look empty, investigate"
• It should pick an investigation directory, confirm the symptom against live data before theorizing, and raise a DEV: task for anything it cannot reach (corpus MySQL needs VPN, for instance) rather than stopping to wait
• Check the FINDINGS.md it writes, where every number should be reproducible from the queries saved alongside it