P5: light-tier canary — gadfly-shaped reviewer on executus core

examples/reviewer proves the core is sufficient for a static-binary light host
(gadfly's shape) with NO batteries:
- config.Env + model.Configure  -> env-driven model fleet + tier overrides
- model.ParseModelForContext    -> tier resolution + failover
- fanout.Run (PerKey caps)      -> N models x M lenses swarm, per-provider bound
- model.GenerateWith[T]         -> structured findings per (model, lens) cell
- Consolidate                   -> one verdict-led report section per model

Hermetic test runs the full 2x3 swarm against majordomo's fake provider and
asserts the consolidated verdicts. A go list -deps CI check asserts the canary
imports ZERO batteries (the light-tier invariant) — gadfly's go.sum stays free
of gorm/redis/discordgo/sqlite. README + docs updated.

This is the canary; migrating the LIVE gadfly repo onto executus core is a
follow-up (kept separate to not destabilize the active reviewer).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -48,6 +48,9 @@ bot) — mort and gadfly are the first two consumers (heavy and light). See
 - `config/`, `deliver/`, `identity/` — host seams (config / output / identity),
   each with a shipped default.
 - `dispatchguard/`, `pendingattach/` — run-safety primitives.
+- `examples/reviewer` — a **gadfly-shaped PR reviewer on the core only** (env-config
+  model fleet → `fanout` N×M swarm → `model.GenerateWith[T]` structured findings →
+  consolidation), the light-tier canary; CI asserts it pulls in no battery.
 
 ## Design