feat(plan-cli): scaffold strict JSON-first CLI package

[Delqhi]

Scope

Port the existing algorithmic scripts into reusable runtime modules.

Reuse targets

• scripts/plan_validate.py → validation + quality scoring
• scripts/dag_builder.py → dependency graph / critical path / execution waves
• scripts/monte_carlo.py → PERT simulation and confidence intervals

Acceptance criteria

• core modules are importable from the package
• CLI delegates to the runtime modules, not the other way around
• outputs are deterministic and test-covered
• no legacy script logic remains duplicated