refactor: adopt effect-operator-kit behind golden contract tests

[solomonneas]

Adopts effect-operator-kit for the duplicated operator skeleton, behind golden contract tests written before any conversion. Same recipe as lidless-labs/n8nctrl#7.

Order of operations

1. Golden contract tests first: programmatic CLI contracts (run() rejects with original error identity on client construction failure), confirm-gated writes make zero client calls, SDK-owned auth and transport pinned unchanged (the kit HTTP layer was deliberately not adopted here), existing retry helper semantics pinned exactly, CLI exit codes and stderr text, result shape stability.
2. Kit installed from GitHub (prepare builds on install), then adoption slices with every exported name, signature, and observable behavior frozen. Where kit semantics differ from this repo, the repo behavior wins via wrapping.
3. Review pass before merge, findings fixed with exact regression tests: pinned the write-gate error text and gate order, and replaced parse-then-assert golden checks with exact raw payload text pins so serialization drift is visible.

Verification

• Full suite, typecheck, and build green at every step
• Zero golden-test failures across the adoption