fix: Atuin Hex fails to init on bash and zsh

[BinaryMuse]

Fixes #3340

[greptile-apps]

Greptile Summary

The bash/zsh init script used ${{ and }} (format! escape sequences) inside a raw string literal (r#"..."#), which meant the double braces were emitted verbatim to the shell — causing a syntax error. The fix replaces them with standard ${...} bash expansion syntax. A regression test is also added.

Important Files Changed

Filename	Overview
crates/atuin-hex/src/lib.rs	Fixes invalid ${{ double-brace syntax in bash/zsh init script (a raw string literal, not a format! call), replacing with correct ${ bash expansion. Adds a regression test.

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