chore(spec): mark all 004-providers-repository-split tasks complete

[marcusburghardt]

Summary

Marks all 54 tasks in `specs/004-providers-repository-split/tasks.md` as complete now that the full migration has landed:

• T038: `replace` directive removed from `complytime-providers/go.mod`; complyctl pinned to pseudo-version `v1.0.0-alpha.0.0.20260423124442-eafccc5f4f2b` (complytime-providers PR Add the codeowners file #4).
• T051: `SC-005` confirmed — complyctl e2e-test passes on main post-merge.
• T052: `SC-006` confirmed — provider binary discovery works correctly (e2e CI validates end-to-end invocation).
• T054: `SC-008` confirmed — complyctl CI pipeline passes cleanly on main with no provider source directories present.

Also closes issues resolved by the migration:

• Closes Remove OpenSCAP and Ampel plugins from complyctl repository #483 (OpenSCAP and Ampel plugins removed from complyctl)
• Closes Make sure complyctl and new repo both working properly #457 (both complyctl and complytime-providers working properly)

Related Issues

• Closes Remove OpenSCAP and Ampel plugins from complyctl repository #483
• Closes Make sure complyctl and new repo both working properly #457
• Separate ampel and openscap plugins #447

Review Hints

• This PR is a single-file documentation change — only `specs/004-providers-repository-split/tasks.md` is modified (four checkboxes flipped from [ ] to [x]).
• All substantive implementation changes landed in PR feat(provider): rename pkg/plugin to pkg/provider and sweep provider terminology #474 (complyctl) and complytime-providers PRs CPLYTM-356 Initial documentation for the project #2 and Add the codeowners file #4.

[marcusburghardt]

This is just to close the initial spec. I will create others for e2e and spec file.

[gvauter]

LGTM