refactor(connectors): drop dead CLI command field after pi-mono migration

The CLI-adapter era `connector.command` field became vestigial once every
provider dispatch went through pi-ai. The schema still required a non-empty
command, so onboarding saveConnector silently rejected every Pi Mono row
(which carries an empty default). Remove the field from the payload chain
(renderer → IPC schema → DB CRUD → run dispatch → worker job), narrow
RunJob.taskId to match EnqueueRunPayload, and drop the dead helper plus
its test. Schema column stays NOT NULL for DB compatibility; seedConnectors
writes a literal empty string so no migration is needed.

tests/unit/model-dispatch.test.ts

@@ -52,7 +52,6 @@ function makeConnector(id: string): ConnectorRecord {
     label: id,
     description: "",
     status: "ready",
-    command: "",
     version: null,
     authState: "ok",
     lastCheckedAt: null,