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>

examples/reviewer/README.md

@@ -0,0 +1,38 @@
+# examples/reviewer — the light-tier canary
+
+A **gadfly-shaped adversarial PR reviewer built on the executus core only** — no
+batteries, no database, no host adapters. It exists to prove that the core is
+sufficient for a static-binary light host (gadfly's shape), and that such a host
+keeps a `go.sum` free of `gorm`/`redis`/`discordgo`/`sqlite`.
+
+What it exercises, all from core:
+
+| Concern | executus core piece |
+|---|---|
+| Env-driven model fleet + tier overrides | `config.Env` + `model.Configure` |
+| Tier resolution + failover | `model.ParseModelForContext` |
+| N models × M lenses swarm | `fanout.Run` (with `PerKey` per-provider caps) |
+| Structured findings per cell | `model.GenerateWith[T]` |
+| One report section per model, worst-verdict-led | `Consolidate` (local) |
+
+## Run
+
+```sh
+REVIEWER_MODELS=fast,thinking \
+ANTHROPIC_API_KEY=sk-... \
+go run ./examples/reviewer -diff "$(git diff HEAD~1)"
+```
+
+Config (all optional, `REVIEWER_`-prefixed env):
+
+- `REVIEWER_MODELS` — csv of tier names / `provider/model` specs (default `fast`)
+- `REVIEWER_MODEL_TIER_<NAME>` — override a tier's resolved spec
+- `REVIEWER_MAX_CONCURRENT` — total in-flight swarm cells (default 6)
+- `REVIEWER_PROVIDER_CONCURRENCY` — per-provider cap (default 3)
+
+## Test
+
+`reviewer_test.go` runs the whole swarm against majordomo's fake provider
+(hermetic, no network) and asserts the consolidated verdicts. A `go list -deps`
+check in CI confirms the package pulls in no battery and no DB driver — the
+light-tier invariant.