feat(concurrency): provider-wide lens budget, drop the model cap

Concurrency was two multiplicative gates in two processes: entrypoint.sh
capped MODELS-at-once per provider (GADFLY_PROVIDER_CONCURRENCY) while each
model's binary separately capped its own lenses (GADFLY_LENS_CONCURRENCY).
A model therefore held its whole model-slot until its LAST lens finished,
stalling the next model even with idle lens capacity.

Collapse to one throttle: a provider-wide lens budget shared across all of
that provider's models. entrypoint now runs every model in a lane at once and
seeds a single cross-process permit pool per lane (a dir of N flock files,
sized by GADFLY_PROVIDER_LENS_CONCURRENCY -> GADFLY_LENS_CONCURRENCY). Each
lens pass (review+recheck) acquires a permit before it runs and releases it
after, so a model winding down immediately yields its freed permits to
another model's queued lenses. flock auto-releases on process death, so a
killed/crashed model can't leak budget.

- cmd/gadfly/lenssem.go: the flock permit pool (+ lenssem_test.go).
- main.go: runSpecialists holds a shared permit per lens; fanout sized to the
  budget so a lone model can use all of it. Falls back to the in-process limit
  when no pool is set (local runs, tests).
- entrypoint.sh: drop provider_cap/DEFAULT_CONC; run_lane runs all models and
  seeds the per-lane pool.
- GADFLY_PROVIDER_CONCURRENCY / GADFLY_CONCURRENCY are now ignored; the
  reusable workflow marks provider_concurrency deprecated and stops forwarding
  it. Docs (README, CLAUDE.md, examples) updated per the maintenance rule.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

CLAUDE.md

@@ -57,7 +57,7 @@ Dockerfile             multi-stage; private-module creds via BuildKit secrets ne
 .gitea/workflows/build-image.yml   push main → :latest; tag v* → :<tag>+:latest; PR → build-only
 .gitea/workflows/review-reusable.yml  reusable (workflow_call) review job; resolves swarm config at
                        RUNTIME: consumer `with:` input → owner user-scope var (GADFLY_DEFAULT_MODELS /
-                       _SPECIALISTS / _PROVIDER_CONCURRENCY / _PROVIDER_LENS_CONCURRENCY, +
+                       _SPECIALISTS / _PROVIDER_LENS_CONCURRENCY, +
                        GADFLY_ENDPOINT_RAGNAROS) → image default. Vars are injected per-run, so editing
                        one var retunes the whole fleet even though long-lived act_runners CACHE this file
                        by ref (a moved tag is NOT re-fetched — only a runtime value or a fresh @<sha>
@@ -154,10 +154,19 @@ are actually exercised. OpenAI/Anthropic/Google come from majordomo's abstractio
   NOT re-pulled, so the job silently runs the previous image. For a run that must use a specific
   build (e.g. validating a just-pushed fix), pin the consumer stub to the immutable
   `:sha-<short>` tag the build publishes, not `:latest`.
-- **Concurrency is per-provider** (`entrypoint.sh`): each provider is a lane, lanes run in
-  parallel, `cap` (from `GADFLY_PROVIDER_CONCURRENCY` else `GADFLY_CONCURRENCY`, default 1) bounds
-  models-at-once within a lane. The review timeout (`GADFLY_TIMEOUT_SECS`) is **per-lens**, not
-  shared across the suite — a slow model can't starve later lenses (the original timeout bug).
+- **Concurrency is one per-provider lens budget** (`entrypoint.sh` + `cmd/gadfly/lenssem.go`):
+  each provider is a lane, lanes run in parallel, and within a lane ALL of the provider's models
+  run at once — the only throttle is a **provider-wide lens budget** (max lens passes in flight,
+  a lens = one specialist's review+recheck). The budget comes from
+  `GADFLY_PROVIDER_LENS_CONCURRENCY` (`provider=N` map) else `GADFLY_LENS_CONCURRENCY` (default 1).
+  Because models are separate processes, the budget is a **cross-process permit pool**: entrypoint
+  seeds a per-lane dir of N flock files; each model's binary acquires one before a lens pass and
+  releases it after (flock auto-drops on process death). This replaced the old two-level
+  `GADFLY_PROVIDER_CONCURRENCY` MODEL cap × per-model `GADFLY_LENS_CONCURRENCY`, which multiplied
+  and let a model hold its slot through its last lens — stalling the next model with idle lens
+  capacity. Those two model-cap vars are now **ignored**. The review timeout
+  (`GADFLY_TIMEOUT_SECS`) is **per-lens**, not shared across the suite — a slow lens can't starve
+  the others (the original timeout bug).
 - **Large-PR token burn**: the agent loop re-sends the whole transcript every step, so a giant
   diff (the old `get_diff` dumped it untruncated, and it was embedded in both the review and
   recheck task) was re-transmitted ~steps × lenses × passes × models times — a ~250 K-token PR