[josegonzalez]

The new sdk/dokkutest package is a stateful fake dokku. It holds apps, config, domains, ports, buildpacks, process scale, plugin properties and datastore services with their info and links, answers the commands the core tasks send in the formats their probes parse, applies their changes, records every call, and fails any command it does not model with an error naming it and a failed test. It covers dokku_app, dokku_config, dokku_domains, dokku_ports, dokku_buildpacks, dokku_ps_scale, every property-table dokku_*_property task, dokku_service_create and dokku_service_link, plus the version gate and the parent app or service probes. Handle, HandlePrefix and Fallback answer anything else, and Exit and Unavailable fail a command the way dokku or a missing binary would.

A test drives each covered type through create, refresh (ReadTask and ReadAddress) and destroy, asserting on the resulting state, and a guard test plans, applies, reads and exports every catalog example of every covered task against a seeded fake, failing on any command it does not model. The Makefile and CI test lines now cover ./sdk/..., and the "Testing without a server" section of docs/embedding.md and the sdk package doc describe the fake.

The second commit moves docket's own fakes onto it: the sdk tests and examples that scripted dokku's answers, and the internal/tasks observed fixtures for the tasks the fake models. Failure cases answer the one command they fail with a handler. The observed fixtures for tasks the fake does not model keep their scripted responses, and ExampleContextWithRunner keeps its hand-written runner since it documents that seam.

The fake's engine lives in internal/fakedokku, importing only internal/subprocess, and sdk/dokkutest aliases it and reads the property tables from sdk.Catalog(). A package that imports sdk cannot be used by the in-package tests of internal/tasks without an import cycle, so the engine could not sit in sdk/dokkutest itself. A missing app answers with dokku's exit code 20, as the dokku 0.38.31 devcontainer does, and the fake adds no domains or links on its own.

Closes #650