refactor(op): Unified Method type, ReceiverRegistry, severed starlark (#264) - #269

…#264)

Phase 5+6: sever starlark from pkg/op, consolidate registries, unify
method dispatch.

Method type (pkg/op/method.go):
- Unified action implementation replacing actionBase, action,
  fallibleAction, compensableAction
- Implements Action and CompensableAction interfaces
- DoFunc/UndoFunc closures set by bind during registration
- MethodKind enum: pure, fallible, compensable
- Shared between graph execution and starlark bridge construction

ReceiverRegistry (pkg/op/registry.go):
- Stores factories (by receiver name) and Methods (by action name)
- RegisterMethod for *Method entries
- Legacy Register path wraps handwritten actions into *Method
- Method() for typed lookup alongside Get() for Action interface

ReceiverFactory interface:
- MethodParams()/MethodParamsFor() on the interface
- Register(ctx, registry) — ctx first
- Generated receivers: single file, inline params, no params.gen.go

Starlark severed from pkg/op:
- Context.Thread: *starlark.Thread → any; bind.ThreadFrom() accessor
- ResourceBase.MarshalStarvalue removed; ID()/OriginID() exported
- PlanningReceiverFactory/ExecutingReceiverFactory moved to bind
- Override removed from both receivers

Code generator:
- params.gen.go.template deleted, params inline in receiver template
- *args/**kwargs inferred from Go types (args []T, kwargs map[string]any)
- ResolveAttr in SKIP_METHODS
- dependent_type template updated
- Register(ctx, registry) signature

Other:
- ActionRegistry renamed to ReceiverRegistry throughout
- RegisterReceiverParams calls removed from generated Register methods
- Flow integration tests use plangen.Receiver
- Test mocks updated for new ReceiverFactory interface