feat(skills): tutorial-qa-to-prod author skill (#2310)

[jung-thomas]

What

Adds a project-scoped Claude skill tutorial-qa-to-prod so every tutorial author can promote a QA-previewed tutorial to PROD with a single natural-language request.

Closes #2310.

Flow it automates

Author previews a tutorial on the QA channel (/tutorials-qa/<slug>, served from the private sap-tutorials/<product>-Contribution repo). The skill copies that tutorial markdown to the canonical public sap-tutorials/Tutorials repo (PROD source) at the same tutorials/<slug>/<slug>.md path and opens a cross-linked PR in each repo. Merging the Tutorials PR triggers the PROD content rebuild.

Correctness details (verified 2026-09-15 against live repos)

• Resolves each repo's default branch dynamically — Tutorials=master, <product>-Contribution=main today. Does NOT hardcode (the older dual-pr skill assumed both master, now stale for Contribution repos).
• Handles CREATE vs UPDATE — first promotion of a QA-only tutorial 404s in PROD; the contents PUT omits sha to create.
• Drift guard on UPDATE — diffs current PROD against QA before overwriting; empty diff = no-op (no PR), non-empty = confirm it's only the intended change.
• Byte-for-byte verify via authenticated contents API (private-repo safe) before opening any PR.
• Two separate PRs (no shared fork network); never targets a non-default base; never force-pushes.

Relationship to existing skills

• sap-tutorial-dual-pr — edits a tutorial and mirrors the edit to both repos in lockstep.
• tutorial-qa-to-prod (this) — promotes a QA-approved tutorial to PROD (create-or-update + drift guard).

Read-only prescribed gh api calls dry-run cleanly against btp-adai-Contribution → Tutorials (create path exercised).