feat: add --version flag to CLI (#12)

## Summary
- Adds `version` to the `#[command(...)]` clap attribute so `aittributor
--version` prints the version from `Cargo.toml`
- One-line change in `src/main.rs`

## Test plan
- [x] Verified `aittributor --version` outputs `aittributor 0.4.0`
- [x] Temporarily changed version to `99.99.99` and confirmed output
updated accordingly
- [x] Reverted version back to `0.4.0` and confirmed correct output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: baldawarishi

src/main.rs

@@ -12,7 +12,7 @@ use agent::Agent;
 use git::{append_trailers, find_git_root};
 
 #[derive(Parser)]
-#[command(name = "aittributor")]
+#[command(name = "aittributor", version)]
 #[command(about = "Git prepare-commit-msg hook that adds AI agent attribution")]
 struct Cli {
     /// Path to the commit message file