feat: add harness-agnostic ytstudio agent skill

[jdwit]

Closes #29.

Authors and publishes a harness-agnostic ytstudio agent skill, following the open SKILL standard. Per the issue discussion, it lives in this repo at skills/ytstudio/ (no separate repo), and the bundled reference is generated from the CLI so it cannot drift.

What's included

skills/ytstudio/
├── SKILL.md                          # frontmatter + concise instructions
├── references/
│   └── reference.md                  # full command reference (generated, committed)
└── assets/
    └── upload-sidecar.example.yaml   # example upload sidecar

• SKILL.md — name: ytstudio / one-line description of when to use it. Body uses progressive disclosure: prerequisites (install, one-time OAuth, profiles), the two rules an agent must know (-o json for parseable output; mutations are dry-run until --execute), per-command-group usage, quota awareness, and a recommended agent workflow. Points at references/reference.md rather than dumping every flag. No MCP, nothing harness- or vendor-specific.
• references/reference.md — the full command reference, generated from the live typer app.
• assets/upload-sidecar.example.yaml — example sidecar mirroring the videos upload schema.

Keeping the reference in sync

docs/reference.md is built on the fly by mkdocs and git-ignored; the skill copy is checked in (it travels with the skill), so it can drift. Two guards, both from the same typer source as the docs:

• uv run python scripts/build_skill_reference.py regenerates it.
• tests/test_skill_reference.py re-renders and fails if the committed copy is stale, so the existing pytest CI job catches drift.

Docs

• README gains an Agent skill section linking the skill and the sync docs.
• New docs/agent-skill.md page (added to the mkdocs nav) explains usage and the sync workflow.

Verification

• uv run pytest — 254 passed (includes the new drift test).
• uv run ruff check / ruff format --check — clean.
• uv run mkdocs build --strict — clean (no broken links).
• Independent adversarial review cross-checked every command/flag/enum in SKILL.md against the typer dump and source, validated the frontmatter against the spec, and checked the sidecar against upload_pipeline.py: no defects.

Follow-up (not in this PR)

Listing on skills.sh is an external action: once merged, point the registry at the skills/ytstudio/ subpath.