feat: configurable lens fan-out, per-provider like model concurrency

Specialist lenses ran strictly sequentially within a model. Add a
GADFLY_LENS_CONCURRENCY knob (default 1 = unchanged) that overlaps the
independent per-lens review+recheck passes, so a model posts its
consolidated comment as soon as its lenses finish.

Per-provider configurable, mirroring GADFLY_PROVIDER_CONCURRENCY:
GADFLY_PROVIDER_LENS_CONCURRENCY takes a "provider=N,..." map keyed by
the same provider lanes (modelProvider() mirrors entrypoint's provider_of;
providerOverride() mirrors provider_cap). The override wins for the model's
lane, else the scalar default.

runSpecialists fans out via a bounded worker pool, order-preserving
(results written by index) and keeping each lens's own timeout/recheck.
repoFS is immutable + fresh-toolbox-per-pass, so lenses share no mutable
state (verified under -race). Docs/examples updated; dropped a duplicate
GADFLY_TIMEOUT_SECS README row.

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

examples/adversarial-review.yml

@@ -55,6 +55,16 @@ jobs:
           # csv to choose; "all" for everything; or define custom ones via a repo
           # .gadfly.yml / GADFLY_SPECIALIST_<NAME>. See README "Specialists".
           GADFLY_SPECIALISTS: ${{ vars.GADFLY_SPECIALISTS }}
+          # Lens fan-out (optional; default 1 = lenses run sequentially within a
+          # model). Raise it to run a model's lenses concurrently so each model
+          # posts its comment sooner. Total in-flight requests = (models at once)
+          # × (lenses at once), so to fan out without oversubscribing a backend,
+          # keep its model cap low and raise its lens cap. Per-provider configurable
+          # via GADFLY_PROVIDER_LENS_CONCURRENCY (same lanes as the model map):
+          #   GADFLY_PROVIDER_CONCURRENCY:      "ollama-cloud=1,m1=1"
+          #   GADFLY_PROVIDER_LENS_CONCURRENCY: "ollama-cloud=3,m1=1"
+          # GADFLY_LENS_CONCURRENCY: ${{ vars.GADFLY_LENS_CONCURRENCY }}
+          # GADFLY_PROVIDER_LENS_CONCURRENCY: ${{ vars.GADFLY_PROVIDER_LENS_CONCURRENCY }}
           # --- Models & providers (optional; default = Ollama Cloud) ----------
           # Gadfly is majordomo-powered, so it can target other backends. Set a
           # provider for bare model ids; point at a different endpoint with a