feat(agents): pin explicit model on every agent and workflow call

Agents and workflows previously carried no model field, so every
dispatch inherited the session model — including fable, which is
banned for all plugin agents and workflows by owner decree. Every
dispatch now pins opus or sonnet explicitly.

- opus (low-volume judgment gates whose misses silently poison
  downstream work): architect, bencher, debugger, fieldtester,
  grounding-check, plan-recon, quality-reviewer, spec-skeptic,
  tdd-author
- sonnet (mechanical scope, in-loop or fanned out): docwriter,
  glossary-extractor, implementer, spec-reviewer, synthetic-user,
  tester
- workflows: all 13 agent() call sites pin a model — sonnet
  everywhere except the quality-reviewer gate in implement-loop,
  the loop's last correctness check (spec-reviewer only gates
  task-text correspondence; real-bug finding is the documented
  opus strength)
- docs/agent-template.md: model is now a mandatory frontmatter
  field, with the assignment rule and the fable ban recorded

tdd/agents/tdd-author.md

@@ -2,6 +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
 ---
 
 # tdd-author