[T3] Correct approval run identity from envelope RunID

[dennisonbertram]

[parent] #1339

Problem

Approvals posted against wrong run: handlers ignore envelope msg.RunID (model.go ~4683, ~4700).

Outcome

Use envelope msg.RunID with fallback in tool.approval_required / plan.approval_required handlers.

Acceptance criteria

• External continuation run approval hits correct run.

Test-first plan

Cross-run approval identity test (expected red).

Impact analysis

TUI-only.