Simplify README agent skill placement

[yolo-sam]

Summary

• add root SKILL.md as the canonical public skill source
• keep .agents/SKILL.md as the local-agent mirror and copy root SKILL.md into dist/html-docs/SKILL.md during builds
• simplify README install flow with the optional agent skill command and root manual link
• switch README/package release-check commands from npm to Bun, while keeping the public Skills CLI example as npx

Verification

• bun run release:check (first run hit stale live Obsidian tab count; clean rerun passed)
• diff -u SKILL.md .agents/SKILL.md && diff -u SKILL.md dist/html-docs/SKILL.md
• git diff --check
• private-path grep over public docs/source
• GitHub CI build-and-lint passed on Node 20.x and 22.x