feat: toggle the enabled state of workers

[NickJosevski]

Stacked on #694 — merges into nj/issue-311, not main. Review #694 first.

Answers the "should workers get the same treatment?" question on #694: yes.

What changed

Refactor first — the enable/disable flow moves from pkg/cmd/target/shared/disabledstate.go into pkg/machinescommon, behind a Machine interface and a MachineKind that describes one machine repository. The noun in the prompt, the "already enabled" message and the "no ... to disable were found" error all come from the kind, so there is one copy of the flow instead of one per machine type. Deployment targets keep exactly the behaviour and output they had.

New commands — octopus worker enable [<name> | <id>] and octopus worker disable [<name> | <id>], mirroring the deployment-target pair: prompts when no identifier is given (offering only workers not already in the requested state), short circuits without an update call when the worker is already in that state, errors under --no-prompt when no identifier is supplied.

New create flag — --disabled on worker listening-tentacle create and worker ssh create (the two worker types with a create command), included in the generated automation command. Flag-only, no prompt — same as the target side.

--disabled's help text is now parameterised by noun, the way RegisterCreateTargetProxyFlags already is.

Test evidence

• pkg/machinescommon/disabledstate_test.go — the prompt flow against a fake Machine, covering both nouns; replaces the old pkg/cmd/target/shared/disabledstate_test.go.
• pkg/cmd/worker/enable/enable_test.go, pkg/cmd/worker/disable/disable_test.go — end-to-end through the root command against the mock HTTP server: named worker, already-in-state short circuit, interactive prompt. The PUT body is asserted to carry the flipped IsDisabled.
• pkg/cmd/worker/worker_test.go — enable/disable registered, and both worker create commands expose --disabled.
• test/integration/worker_test.go — mirrors the deployment-target integration test, including that a toggle leaves the worker's other settings untouched.

go build ./... clean; go test ./pkg/... all pass. Both integration tests (TestWorkerEnableDisable and the existing TestDeploymentTargetEnableDisable, to check the refactor) run green against a local Octopus instance. gofmt/go vet report only pre-existing issues in files this PR does not touch.

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