mnemo 1.2.0 — universal-executor gate

Opt-in universal-executor gate on spend_irreversible.

• is_universal_executor(tool, signature) — detect verb-polymorphic shell/eval/SQL/HTTP tools whose reversibility is not decidable from the signature.
• spend_irreversible(tool=, contained=) — an uncontained universal executor is denied outright (reversibility undecidable + unbounded external harm-reach); contained=True falls through to the normal per-source budget. tool=None is byte-identical to 1.1.0.

Motivated by a measured lab result (ToolEmu 330 tools, 2 labelers: reversibility ~93% signature-decidable, Cohen's kappa 0.82; the ~7% residual is universal executors with environment-conditional harm-reach). Credits arXiv:2607.07474. 27 tests pass.