feat(models): add fable tier for the four singleton judgment gates

Owner-ratified 2026-07-20: architect, grounding-check, tdd-author and
debugger pin fable/xhigh — cycle-reach singleton gates whose miss
silently ratifies or poisons an entire cycle (drift verdict, autonomous
sign-off signature, RED executable-spec, root-cause). The set is
closed: extending it requires fresh owner ratification, no fable agent
ever runs per-task in a loop or in a parallel fan-out, and workflow
scripts stay fable-free (the implement-loop quality gate caps at opus).

Policy home: docs/agent-template.md $ model rule 1 (effort: fable
gates stay xhigh, not max); the workflow policy headers now point at
the exception instead of the former blanket ban.

implement/workflows/implement-loop.js

@@ -80,7 +80,8 @@
 //     the end-report for the orchestrator to write.
 //   • Model policy: every agent() call pins an explicit model — never the
 //     session-model inherit (which could route to an unintended tier, e.g.
-//     fable — banned for all plugin agents and workflows). Mechanical and
+//     fable — reserved for the four ratified frontmatter gates, never for
+//     workflow stages; agent-template § model rule 1). Mechanical and
 //     in-loop steps run sonnet; the quality gate runs opus by default (last
 //     correctness review before the end-verify), TIERED to sonnet/high for a
 //     small (<= 25 changed lines), non-contract diff as measured