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

implement/agents/implementer.md

@@ -2,6 +2,7 @@
 name: implementer
 description: Carries out a tightly scoped implementation task. Reads the task extract handed by the controller, implements, builds, tests, reports a structured status with the diff. NOT for architecture decisions, NOT for self-curated scope; this agent executes a plan that has already been made.
 tools: Read, Edit, Write, Bash, Glob, Grep
+model: sonnet
 ---
 
 # implementer

implement/agents/quality-reviewer.md

@@ -2,6 +2,7 @@
 name: quality-reviewer
 description: Read-only code-quality reviewer for project diffs. Reports Strengths, Issues by severity (Important / Minor / Nit), and a Recommendation. Runs after spec-reviewer is green; spec-compliance is NOT this agent's concern. Does NOT propose fixes.
 tools: Read, Glob, Grep, Bash
+model: opus
 ---
 
 # quality-reviewer

implement/agents/spec-reviewer.md

@@ -2,6 +2,7 @@
 name: spec-reviewer
 description: Read-only spec-compliance reviewer. Compares a recent diff against the task text from a plan under docs/plans handed by the controller. Reports missing requirements and unrequested extras. Does NOT review code quality (that is quality-reviewer's job) and does NOT propose fixes (the implementer fixes; the orchestrator coordinates).
 tools: Read, Glob, Grep, Bash
+model: sonnet
 ---
 
 # spec-reviewer

implement/agents/tester.md

@@ -2,6 +2,7 @@
 name: tester
 description: Writes new fixtures and E2E tests after a cycle or feature ships. Verifies a feature works from build through to observable output. Each test protects a named property; tests check observable behaviour, not implementation internals.
 tools: Read, Edit, Write, Bash, Glob, Grep
+model: sonnet
 ---
 
 # tester