fix: make the in-process execution mode explicit in specs and contract checks

Two coupled holes surfaced in the Phase 0-3 review. duet--transfer-spec copied only :argv, :default-directory, and :process-environment out of a backend's command result, dropping the :tramp marker, so a TRAMP-routed spec arrived as :argv nil with nothing telling the executor to copy in process. And duet--check-command leaned on listp, where nil is a list in Elisp, so a command builder returning nil or a bare :argv nil passed the minimum tier.

Both turn on the same idea, so they share a fix. duet--command-spec-executable-p defines a runnable spec: a non-empty plist with either a non-empty :argv of strings (a CLI backend) or a declared in-process mode such as :tramp. The contract checker rejects anything else, and transfer-spec now carries :tramp through, so the TRAMP fallback has a positive execution signal rather than an ambiguous nil argv.

The legitimate TRAMP backend keeps passing because it declares its mode. A broken backend that forgets argv no longer slips through.

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