fix: Don't run 'atuin init' in 'atuin hex init' — each must be initialized separately

[BinaryMuse]

This PR separates atuin init from atuin hex init, allowing for configuration flags to be passed to atuin init.

Fixes #3329
Fixes #3323

[greptile-apps]

Greptile Summary

Decouples atuin hex init from atuin init so each must be initialized separately, allowing flags to be passed to atuin init. The generated shell scripts no longer embed eval "$(atuin init …)" calls, and a new reference doc explains the required two-step setup.

• Shell::as_str and init_command helpers removed; tests updated to assert the absence of the old embedded atuin init calls.
• New docs/docs/reference/hex.md documents the separate initialization requirement with per-shell examples.
• Minor doc nit: stray **bash** on line 16 of hex.md will render as extraneous bold text above the tab block.

Important Files Changed

Filename	Overview
crates/atuin-hex/src/lib.rs	Removes Shell::as_str and init_command helpers, strips the eval "$(atuin init …)" lines from the generated shell scripts, and updates tests to assert absence of those lines. Logic is correct.
docs/docs/reference/hex.md	New reference page for atuin hex init. Minor issue: stray **bash** line (line 16) above the tabbed code block will render incorrectly.
docs/mkdocs.yml	Adds hex: reference/hex.md entry to the nav in alphabetical order. No issues.

_{Reviews (1): Last reviewed commit: "docs: Add 'atuin hex' docs" | Re-trigger Greptile}

[ellie]

looks good and solves our current issues but i REALLY want the hex init to be much much easier than this. no ifs or thinking required for the user to get it running