chore(ci): add /prerelease skill for tagging alpha/beta/rc releases

[brayniac]

Summary

• Adds .claude/skills/prerelease/SKILL.md — a Claude Code project skill invoked as /prerelease
• Tags and pushes the current Cargo.toml version to upstream as a prerelease
• Guards: aborts if the version has no - suffix (stable versions use the tag-release CI workflow instead), aborts if the tag already exists on upstream, and requires a clean working tree on main

Test plan

• Run /prerelease on a branch with a stable version string — should abort with a clear error
• Run /prerelease when the tag already exists on upstream — should abort
• Run /prerelease on a dirty working tree — should abort
• Run /prerelease normally on main with an alpha version — should tag and push, triggering the release.yml pre-release build

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