Quick wins: LazyLock regex, body default, document github-host, remove get_tag

[dexwritescode]

Summary

• Check GITHUB_TOKEN env var #6 Regex LazyLock — semver regex is now compiled once via std::sync::LazyLock instead of on every from_str call; Rust bumped to 1.95.0 to support it
• Create writer module #8 body unwrap_or_default — get_body() no longer calls exit(1) when INPUT_BODY is unset; uses unwrap_or_default() since the input is optional with a '' default
• Use slim-bookworm debian/rust release #7 Document github-host — github-host is now a declared input in action.yaml, making GitHub Enterprise support visible to users (the binary already read INPUT_GITHUB-HOST)
• #5 Remove redundant get_tag() — Semver::get_tag() was identical to Display/to_string(); removed the method and updated all call sites

Test plan

• cargo test passes (6 tests, all green)
• No logic changes — purely cleanup and documentation

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