Fable Validate Loop

Use when the user asks to validate a GitHub issue with Fable 5.1 and then autonomously drive it to a reviewed PR in one shot — "fable-validate-loop", "fable validate and work on #N", "fully automate issue #N with fable". Runs fable-validate, auto-applies its update-issue edits when the verdict calls for it, has fableplan produce and post a Fable 5.1 implementation plan (skipped only when the verdict's title-floored signal reads `fableplan: no`, which means the title score and the recomputed score are both below 71, with no safety flags and no `fableplan first: Yes` stamp in the issue's Execution block), then hands off to work-on-issue-loop — stopping instead when validati…