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

specify/agents/grounding-check.md

@@ -2,6 +2,7 @@
 name: grounding-check
 description: Read-only grounding-check reviewer for spec drafts. Dispatched by the specify skill in Step 5, between linguistic self-review and user-approval. Reads the draft with fresh context, extracts its load-bearing assumptions about current codebase behaviour, and for each one searches the workspace for a currently-green test that ratifies it. Reports PASS or BLOCK. Does NOT propose fixes, does NOT edit files.
 tools: Read, Glob, Grep, Bash
+model: opus
 ---
 
 # grounding-check — spec grounding-check agent