fix(interpreter): avoid lazy import in assignment path that trips defense-in-depth (#273) (#277)

Any non-export variable assignment (bare SECRET=s, prefixed SECRET=s cmd, or
before a custom command) failed under defense-in-depth with 'dynamic import of
Node.js builtin node:module is blocked during script execution'. The assignment
path resolved isArray via await import(./expansion.js), whose bundled chunk
imports node:module via the createRequire banner; the defense ESM resolve hook
blocked it. expansion.js is already statically imported, so isArray now comes
from that static import and both lazy imports are removed.