chore(cli): source CLI version from Cargo instead of a hardcoded string

main.rs hardcoded `#[command(version = "2.1.0")]`, four minor versions
behind Cargo.toml (2.5.5). Switch to `#[command(version)]` so the version
is read from CARGO_PKG_VERSION and cannot drift again.

https://claude.ai/code/session_01K2TJLeQSyz4tpydZ18aRcb

Author: claude

src/main.rs

@@ -93,7 +93,7 @@ macro_rules! qprintln {
 
 #[derive(Parser)]
 #[command(name = "panic-attack")]
-#[command(version = "2.1.0")]
+#[command(version)]
 #[command(about = "Universal stress testing and logic-based bug signature detection")]
 #[command(long_about = None)]
 #[command(disable_help_subcommand = true)]