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

docs/agent-template.md

@@ -12,6 +12,7 @@ specific identifiers.
 name: <agent-slug>
 description: <third-person, "Use when…" or role description>
 tools: <comma-separated tool list>
+model: <opus | sonnet>
 ---
 
 > Violating the letter of these rules is violating the spirit.
@@ -105,6 +106,37 @@ dispatch still carry no `Agent` tool — the no-nested-dispatch rule is
 unchanged; the workflow simply does the orchestration the retired
 `implement-orchestrator` agent used to fake with inline role-switches.
 
+### `model`
+
+Mandatory. Every agent pins an explicit model — `opus` or `sonnet`,
+nothing else. An omitted field means the agent inherits the *session*
+model, which routes the dispatch to whatever the user happens to be
+chatting on (possibly `fable` — banned for all plugin agents and
+workflows, by owner decree). Pinning removes that coupling.
+
+Assignment rule, in priority order:
+
+1. **Consequence of a miss** — agents whose errors silently poison
+   downstream work (adversarial gates, root-cause diagnosis,
+   architecture judgment) run `opus`. Opus is the documented
+   strength for code review, debugging, and deep reasoning.
+2. **Mechanical scope** — tightly-scoped execution of a pre-made
+   plan, recon, extraction, compliance-diffing run `sonnet`
+   (near-opus coding quality at lower latency and cost).
+3. **Volume × wall-clock** — agents dispatched per-task inside
+   loops or fanned out in parallel swarms multiply their model's
+   latency; they run `sonnet` unless rule 1 overrides (currently
+   only `quality-reviewer`, the loop's last correctness gate).
+
+Current distribution: `opus` — architect, bencher, debugger,
+fieldtester, grounding-check, plan-recon, quality-reviewer,
+spec-skeptic, tdd-author. `sonnet` — docwriter, glossary-extractor,
+implementer, spec-reviewer, synthetic-user, tester.
+
+The same rule binds Workflow scripts: every `agent()` call passes an
+explicit `model:` option (see the model-policy header comments in
+`implement/workflows/*.js`).
+
 ## Sections in detail
 
 ### Spirit-letter lead-in