insider: move the variants dataset into its own subfolder (complete the multi-dataset layout)

[enriquea]

INSIDER became a two-dataset provider when insider:interfaces was added in #231, but its
layout only half-follows the plugin convention.

hvantk/skills/_conventions/SKILL.md and CLAUDE.md both say:

Multi-dataset providers add one subfolder per dataset plus a shared/ folder.

Current layout:

hvantk/skills/insider/
├── builder.py          <- variants, still at the provider root
├── drift_probe.py
├── plugin.yaml
├── shared/             <- added in #231 (7712b503)
├── interfaces/         <- interfaces, correctly in its own subfolder
└── tests/              <- variants tests, still at the provider root

For comparison, hvantk/skills/cptac/ is the reference and splits cleanly into
expression/, phospho/ and shared/.

What is already done

#231 added insider/shared/ and moved the file:// path normalisation into it, so the
duplication between the two builders is gone — that was the concrete harm the convention
protects against.

What is left

Move the variants dataset into insider/variants/: builder.py, its tests/, and the
builder.module / drift_probe.module paths in plugin.yaml.

Why it was not done in #231

It re-points a working dataset's module paths in the manifest and its tests, mid-review, on
a branch that was already large. That is risk without correctness gain — the dedup fixed
the real problem, and the remaining change is purely structural.

Notes for whoever picks it up

• plugin.yaml resolves builders by dotted module path, so the manifest must change in the
  same commit as the move or the plugin fails to load.
• hvantk/tests/test_plugin_smoke.py asserts the exact set of registered datasets; it does
  not need changing for a move, but it is the test that catches a broken manifest fastest.
• The snapshot fixtures for variants live under the plugin's tests/ and are referenced
  from plugin.yaml's tests: block by relative path — those paths move too.