Adds tests/run-tests.fish (syntax lint over every .fish file, plus a
sandboxed interactive load) and tests/functional.fish (10 checks
covering XDG/PATH/CDPATH setup, key bindings, abbreviations, core
functions, exit rewiring, and the opinionated-component registry).
The sandbox copies config-relevant files into a scratch HOME/XDG tree
rather than symlinking the checkout, since this repo also serves as a
live ~/.config/fish and a symlink would let universal-variable writes
leak into the real fish_variables file.
Wires the suite into build-docs.yml as a `test` job that `build-docs`
now depends on, so a broken config can no longer get published to the
docs site. Documents the workflow in the README's new Testing section.
rootiest
642c981e4b