fix: add --version flag to CLI

[don-petry]

Summary

• Adds #[command(version)] attribute to the Cli struct so that dgr --version prints the crate version from Cargo.toml.

Refs #11 (item 7).

Test plan

• Run cargo run -- --version and confirm it prints the version string
• Run cargo run -- --help and confirm --version appears in the output

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds first-class --version support to the dgr CLI by enabling clap’s built-in version flag on the root Cli parser, so dgr --version prints the crate version from Cargo.toml (refs #11 item 7).

Changes:

• Enable clap’s --version flag via #[command(version)] on the top-level CLI struct.

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