feat(boatstack): introduce deterministic DecisionOperator at plan boundary

[bigboateng]

Decision Architecture: Plan Validation Boundary

This PR introduces the foundational primitive for Boatstack's Decision Architecture.

Instead of hardcoding ValidatePlan to return a string error that triggers a user prompt, Boatstack now acts as a repository supervisor that coordinates coding agents through evidence-backed delivery boundaries.

It evaluates uncertainty at the validateArchitectureGrounding boundary using a deterministic DecisionOperator:

• Infer: Verified repository evidence resolves the decision.
• Query: Material product intent requires human input.
• Verify: Evidence is supported but requires independent verification.
• Reject: Planning premise is not supported.
• Escalate: Repository evidence conflicts or is otherwise unresolvable.

Why this matters

Boatstack is no longer simply asking questions when uncertain; it selects the admissible operator that maximizes progress toward a verified delivery. The decision is backed by a derived EvidenceLevel from the existing ledger and bounded by human authority and safety constraints.

Changes

• Introduced DecisionOperator, EvidenceLevel, PremiseStatus, and ResolvePlanDecision() in decision.go.
• Refactored validateArchitectureGrounding to derive evidence levels and construct a PlanDecisionInput, rather than instantly returning an error.
• Exhaustively tested deterministic branches in decision_test.go and verified integration in plan_validation_test.go.