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.

debug/agents/debugger.md

@@ -2,7 +2,7 @@
 name: debugger
 description: Diagnoses bugs in the project. Reproduces, finds the root cause, writes a RED test that pins down the symptom, and hands off to implement mini-mode for the GREEN side. Does NOT apply the fix itself.
 tools: Read, Edit, Write, Bash, Glob, Grep
-model: opus
+model: fable
 effort: xhigh
 ---