Fish shell users currently fall into the catch-all case, which writes
POSIX export syntax to ~/.profile. Fish does not source ~/.profile and
does not use export PATH="...:$PATH" syntax, so the PATH addition
silently does nothing.

Add a fish case that targets the idiomatic conf.d directory and uses
fish_add_path, matching how the script already handles zsh and bash
with their respective profile files and syntax. Extract the PATH
command into a variable to avoid duplicating the shell-specific logic
across the interactive prompt, non-interactive hint, and get-started
instructions.

Replace scattered rm -rf calls with a single EXIT trap to ensure the
temp directory is always cleaned up on exit. Previously, failures in
curl/wget download, tar extraction, chmod, or mkdir left temp files
behind because set -e would exit before reaching manual cleanup calls.

The trap fires on any exit (success or failure), so it covers all
paths with one line while removing five redundant cleanup calls.

install: use EXIT trap for temp directory cleanup

…l-path

install: add fish shell support for PATH configuration