fix: use printf to append fish shell init block (#3346)

## Summary

- The existing sed-based fish config injection produced malformed output
on some systems, concatenating 'atuin init fish | source' and 'end' onto
the same line (issue #3192)
- Replace the GNU/BSD sed detection block with a single printf append
that writes a self-contained 'if status is-interactive' block
- Remove the now-unnecessary sed dependency check from the installer

## Root cause

sed backslash-newline replacement behaves inconsistently across GNU and
BSD (macOS) implementations, causing the newline to be dropped on some
systems.

## Fix

Use printf to append a new 'if status is-interactive' block, consistent
with how bash/zsh init lines are already added.

## Test plan

- shellcheck -e SC2148 install.sh passes with no warnings
- Idempotency preserved via existing grep -q duplicate guard

Fixes #3192

Signed-off-by: majiayu000 <1835304752@qq.com>

0 files changed, 0 insertions, 0 deletions