The envelope's `type` field was leaking implementation terms ("pruning",
"strict_mode") that describe enforcement mechanism rather than the user-
facing semantic. It also duplicated `detail.layer`, and forced consumers
to branch on two values for the same conceptual error ("a command was
denied by policy").
Collapse both into a single semantic type "command_denied". The
enforcement layer ("pruning" / "strict_mode") is preserved in
`detail.layer` so debugging and per-layer diagnostics still work.
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