v0.4.0: Update Make/CI for relocated Go package layout

[tazarov]

Goal

Update build/test/lint automation to target the new Go subtree layout without breaking existing workflows.

Scope

• Update Makefile targets and path assumptions.
• Update CI jobs to run checks against moved package structure.
• Keep current platform behavior and shim integration unchanged.

Acceptance Criteria

• make test, make lint, and test-all remain green.
• CI workflows pass across configured OS matrix.
• No regressions in release pipeline inputs from layout changes.

[tazarov]

Tracked by umbrella #45 (v0.4.0 root-cleanup plan).

[tazarov]

Completed: Makefile and CI workflows (.github/workflows/ci.yml) updated for the internal/ subtree layout. All targets pass across the OS matrix.