docs: list use-siyuan in README

[vuon9]

What

Adds the new use-siyuan skill to the root README's Written by me list.

Why

PR #2 merged the skill but the root README was not updated, so the skill listing was stale.

Testing

Docs-only change. setup-vstack/scripts/verify.py and the unittest suite are unaffected.

[github-actions]

Skills review — PASS

• PR docs: list use-siyuan in README #3 is docs-only (README.md:55): adds use-siyuan to Written by me to sync with PR feat: add use-siyuan CLI skill #2; no manifest add/remove/re-scope in setup-vstack/skills.json:1-186.
• Required-set matches bidirectionally: setup-vstack/skills.json:17-168 has 7× required:true (brainstorming, wayfinder, test-driven-development, unslop, hunk-review, requesting-code-review, receiving-code-review) ↔ vmode/SKILL.md:33-44 same 7 bullets.
• All 36 manifest entries have valid source (owner/repo or https://github.com/.../tree/... or local) and scope mine/external; every mine → vuon9/*.
• Repo-local skills (vmode/SKILL.md:2, setup-vstack/SKILL.md:2, skills/*/SKILL.md:1-4) — name matches folder/manifest (^[a-z0-9-]+$), description starts Use when, README.md exists next to each SKILL.md.
• No mismatch found; no setup-vstack/scripts/manage.py fix required (e.g. python3 setup-vstack/scripts/manage.py set-required <name> true not needed). Verdict: PASS.

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