fix: force direct tty prompts for zsh hook

[donbeave]

Summary

Fixes the zsh hook path so it explicitly uses Shellfirm's direct /dev/tty prompter instead of relying on stdin().is_terminal() to infer whether requestty/crossterm is safe.

Root Cause

pre-command already has a DirectTtyPrompter fallback for zsh zle contexts, but it is selected only when stdin is not a terminal. In some zsh hook environments stdin still reports as a terminal, so Shellfirm selects TerminalPrompter, enters requestty/crossterm, emits a cursor-position query, and can hang without showing a usable prompt.

Changes

• Adds a hidden pre-command --direct-tty flag that forces DirectTtyPrompter on Unix.
• Updates the generated zsh hook to call shellfirm pre-command --direct-tty -c "${BUFFER}".
• Adds a regression test asserting the generated zsh hook keeps the direct-tty flag.

Fixes #183.

Verification

• cargo test -p shellfirm zsh_hook_uses_direct_tty_prompt
• cargo test -p shellfirm command
• Runtime smoke test with target/debug/shellfirm pre-command --direct-tty -c 'rm -rf /private/tmp/shellfirm-pr-runtime-test', confirming it shows Type 'yes' to continue instead of entering the hanging cursor-query path.