refactor(op): Extract starlark infrastructure from pkg/op into pkg/op/bind #264

Summary

Split pkg/op into a starlark-free core (pkg/op) and a starlark binding package (pkg/op/bind). Create a plan provider (pkg/op/provider/plan) that implements graph-construction as a regular immediate provider. Flatten the plan namespace and eliminate the plan.flow sub-namespace.

Goals

1. pkg/op is starlark-free — zero go.starlark.net imports
2. pkg/op/bind owns all starlark binding infrastructure — runtime, marshal, receivers, promise
3. bind has a minimal public API — bind.StarlarkRuntime and bind.StarlarkConfig only
4. Plan is a provider — pkg/op/provider/plan with pure Go graph-construction methods
5. Flat plan namespace — plan.choose, plan.complete, plan.wait_until etc. at top level
6. No circular dependencies — bind imports op, never the reverse

Phases

1. Create plan provider and flatten plan namespace
2. Create pkg/op/bind package (move starlark binding infrastructure)
3. Sever starlark references from pkg/op
4. Update generated code and code generator
5. Update consumers and verify

Plan

See docs/plans/extract-starlark-from-op.md