Maintaining two nested go.mod/go.sum files required manual sync of
transitive dependencies. Merge the integration-tests module into the
root module to eliminate this pain.

- Delete integration-tests/go.mod and go.sum
- Promote 4 deps from indirect to direct in root go.mod
- Update mise.toml and CI to use ./integration-tests/... package path
- Fix stale comments referencing the old separate module

Signed-off-by: Morgan Fainberg <[email protected]>

The integration test harness was calling 'make wheel' which only builds
the SDK wheel (cog-*.whl) via 'mise run build:sdk', but it checks for
both SDK and coglet wheels. This caused build failures in CI.

Changed to use 'mise run build:wheels' which builds both cog and coglet
wheels, matching what the harness actually checks for.

After consolidating integration-tests into the root Go module, the
grep -v /integration-tests/ filter in test:go missed the root package
(no trailing slash), causing TestIntegration to run in the unit test
job without mise, COG_BINARY, or Docker.

Replace the fragile grep filter with a //go:build integration
constraint on suite_test.go. Unit test jobs now use plain ./... and
integration test jobs pass -tags integration.

Tag login_test.go and concurrent_test.go with //go:build integration
to match suite_test.go. Without this, these tests still compiled in
the unit test job and failed looking for mise.