trail create: bound the Trails check and keep conflicts retryable

Review fixes: read the setting through api.Client.RepoTrailsEnabled with a 5s deadline so a stalled cell falls through to the create, save a definite "off" in the trails-enablement cache, treat 409 as possibly applied (an idempotency-key replay), and keep the --idempotency-key retry hint first. Tests cover the read's outcomes, each refusal status, the hint, and publishing after a failed read.