Onboarding skill for DS4: a docs-grounded agent skill tutor

## Proposal

An agent skill that onboards newcomers to DS4 as a guided tutor, grounding
every answer in the repository's own sources.

### What it does
- Teaches DS4 step by step (what it is → prerequisites → model download →
build → first run → server/agent/bench/eval → cross-cutting concepts →
debugging) following the project's own incremental `--help` philosophy.
- Answers **only** from the certified corpus: `README.md`, `MODEL_CARD.md`,
the `*/README.md` files, and the real CLI `--help` output (source of record
`ds4_help.c` / `ds4_help.h`). No web, no training-data guesses.
- **Every project claim is cited** with a clickable `file:line` plus a GitHub
permalink to the same line.
- Replies in the **user's language**, keeping commands/flags/paths verbatim.
- **Never runs work commands** for the user (no generate/serve/download/build/
bench/eval) — it hands over the exact command to copy. The only execution it
performs is `--help`.

### Why upstream
It keeps newcomers anchored to the real docs and CLI help, and the citation
rule doubles as a lightweight doc-consistency check.

### Open questions
- Preferred location: `.agents/skills/` with `.claude` as a symlink (current),
or another convention?
- Should it live in-repo or in a separate companion repo?

A PR implementing this is linked below.