fix(release): detect published crate versions

[samzong]

Summary

• Add a User-Agent to the crates.io published-version probe so reruns detect already-published crate versions instead of attempting to publish them again.

Scope Check

• Stays within the v0.1 bundled-skill installer boundary.
• Does not add marketplace, registry, remote install, script execution, GUI, MCP server, or agent runtime behavior.
• No golden case update required; this changes release workflow recovery only.
• No spec/hosts.json changes; host constants unchanged.

Verification

• actionlint .github/workflows/release.yml
• YAML parse for .github/workflows/release.yml
• curl -A "kitup-release" -fsS https://crates.io/api/v1/crates/kitup/0.1.3