fix(installer): robust shell detection and fish-safe PATH update

[Dwsy]

Summary

• add robust shell detection for installer using priority: OPENFANG_SHELL -> $SHELL -> parent process -> grandparent process
• fix fish PATH injection to use fish_add_path instead of POSIX export PATH=... syntax
• keep bash/zsh/sh behavior unchanged (export PATH=...)
• fallback profile handling for unknown shells (~/.profile)

Why

When users run curl -fsSL https://openfang.sh/install | sh from fish, shell detection can be ambiguous and PATH lines must be fish-native. Using POSIX export syntax in fish can break PATH and hide core commands.

Validation

• bash -n scripts/install.sh
• fish context check with parent-shell fallback (fish -c, SHELL=/bin/sh)
  • detected shell = fish
  • profile = ~/.config/fish/config.fish
• explicit override checks (OPENFANG_SHELL=fish|zsh)
• end-to-end local install test with isolated HOME:
  • fish writes fish_add_path "<install_dir>" to fish config
  • bash writes export PATH="<install_dir>:$PATH" to bash profile

Scope

No platform download/extract logic changed. Linux/macOS target resolution remains untouched.