[TASK] Enforce internal import for DevsweepCorePrivate

[kirich1409]

Context

In devsweep target, use internal import DevsweepCorePrivate (Swift 5.9+, SE-0409) so composition root is the only public consumer of private implementations.

Acceptance criteria

• main.swift uses internal import DevsweepCorePrivate.
• Negative test: adding import DevsweepCorePrivate inside DevsweepModules fails the build.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

• See research report §Plugin API principle 8.

Dependencies

• Blocked by [TASK] Create Package.swift with 4 targets #19

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved