Add a plan-validation gate for hierarchy, traceability, dependencies, and readiness #1654

Parent epic: #1648

Outcome

Add /squad plan validate as a required gate before acceptance or issue creation.

Validate intent coverage, contradictions/duplicates, unresolved references, hierarchy/orphans, dependency cycles, milestones/metadata, leaf size, independent reviewability, missing decisions, security/operations/migration/test/rollout coverage, and the first execution wave.

Acceptance criteria

• Results contain structured errors, warnings, and advisory findings.
• Errors block acceptance; warnings require acknowledgement or revision.
• Computable checks are deterministic rather than model assertions.
• Validation works on proposed artifacts and existing GitHub issues.
• Regression tests detect Aspiregregator's unresolved temporary dependencies and flat mixed-level backlog.

Size: M