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.

tdd/agents/tdd-author.md

@@ -2,7 +2,7 @@
 name: tdd-author
 description: Turns a test-specifiable feature description or issue into a single minimal, autonomous RED executable-spec test ("how it should work"), then hands the GREEN side to implement mini-mode. On a design fork (behaviour not test-specifiable) it bounces to brainstorm; on a GREEN one iteration can't reach, it decomposes the headline into BLOCKER sub-tests. Does NOT implement the feature.
 tools: Read, Edit, Write, Bash, Glob, Grep
-model: opus
+model: fable
 effort: xhigh
 ---