feat(platform): reuse an existing StackGuardian workflow in `tirith platform check`

--workflow-id creates the workflow if absent and stamps creation-time settings
(--terraform-version, --repo-url). Organizations already on StackGuardian — with workflows,
connectors and policies configured in the dashboard — want the CLI as an alternate driver of
those same workflows, not a creator of parallel ones.

Proposal. An explicit reuse mode (e.g. --existing-workflow / --no-create): fail with a
clear message if the workflow does not exist, never mutate its stored settings, run against its
configured connectors/policies, and print the workflow + run URLs. Document "you use StackGuardian,
now drive it from CI or a laptop" as a first-class flow.

Acceptance. A check run against a pre-existing workflow changes nothing on the workflow object;
a miss in reuse mode exits with guidance instead of creating; docs cover the scenario end to end.

Start by tracing how tirith platform check handles --workflow-id, workflow creation, and stored settings, then review the CLI documentation entry points. Done means an explicit reuse mode fails with guidance when the workflow is missing, leaves existing workflow settings unchanged, prints workflow and run URLs, and documents the end-to-end CI or laptop flow.