Exclude the root-mounted suite from a bare rspec run

[Fivell]

Follow-up to #17, from Copilot's review of it.

.rspec was updated for the engine and isolated-engine suites but never for the root-mounted one that #17 added, so:

--exclude-pattern "spec/{engine,isolated,dummy_engine,dummy_isolated}/**/*"

A bare bundle exec rspec therefore loaded spec/root alongside the default suite and booted spec/dummy and spec/dummy_root in the same process — two Rails applications, one Rails.application.

CI never caught it because CI runs the rake tasks, and Rakefile already had the right exclusion:

t.exclude_pattern = "spec/{engine,isolated,root,dummy_engine,dummy_isolated,dummy_root}/**/*"

This is the same list in .rspec, plus the comment above it updated to name all four suites.

bundle exec rspec with no arguments: 161 examples, 0 failures, 1 pending.