Use unscoped when finding exposed records (#35)

* Use unscoped when finding exposed records

Ensures exposed records can always be found regardless of any
default_scope on the model. For example, if a model uses soft-deletes
with a default_scope filtering out deleted records, the fixture helper
should still return the record since it was explicitly exposed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Add integration test for unscoped record lookup

Adds a soft-delete scenario to the dummy app to verify that exposed
records hidden by a default_scope are still accessible via the fixture
helper. Updates the existing unit test mocks to reflect the new
unscoped call chain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove redundant Repository unit tests

These tests were in a misnamed file (fixture_set_spec.rb for
FixtureKit::Repository) and tested implementation details (memoization,
laziness) with brittle message expectations tightly coupled to the
implementation. The meaningful behaviors are already covered by
integration tests (EXPOSED_ACCESS, ARRAY_EXPOSURE,
UNSCOPED_RECORD_LOOKUP).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove unnecessary migration file

The dummy app sets up tables inline via rails_helper/test_helper,
not through migrations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>