feat(cli): give every preview lifecycle op one JSON document - #3309
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`--status`, `--stop`, `--list` and `--kill-all` emit a schema-versioned envelope with an `ok` discriminant under `--json`, from one writer and one failure-payload builder. Human output is unchanged; the JSON path is additive. The value is in the failure paths. An agent that gets a bare error line on stderr and an empty stdout cannot tell a crash from a "not running", so every failure is a document too — including a missing project, which under `--json` resolves through the throwing resolver rather than the human-shaped nudge.
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--status,--stop,--listand--kill-allemit a schema-versioned envelope with anokdiscriminant under--json, from one writer and one failure-payload builder. Human output is unchanged; the JSON path is additive.{"schemaVersion":1,"operation":"status","ok":true,"result":{"state":"running","mode":"background","port":3002,"pid":1234,...}} {"schemaVersion":1,"operation":"stop","ok":false,"error":{"code":"preview-stop-failed","message":"..."}}Why the failure paths are the point
An agent that gets a bare error line on stderr and an empty stdout cannot tell a crash from a "not running". Every failure is a document too — including a missing project, which under
--jsonresolves through the throwing resolver rather than the human-shaped nudge.Verifying
Start a background preview, then run each of
--status,--list,--stopwith--jsonand pipe stdout through a JSON parser: one document each, with the matchingoperationandstate.Stack
Based on
u4b-preview-session-ownership. Retarget tomainbefore merging.