fix(install): emit fish-native PATH syntax for fish configs (#319)

cbm_ensure_path appended `export PATH="<dir>:$PATH"` to every shell rc,
including config.fish — which is a syntax error in fish and broke users
fish config. Detect a .fish target and emit `fish_add_path <dir>` instead
(idempotent, prepends only if absent). POSIX shells are unchanged.