docs: spec limit semantics, pricing caveats, manifest hedge

I2 (controller ruling): docs/superpowers/specs/2026-08-19-traefik-llmgateway-design.md
described limits as "user overrides group" / "user limit if set, else
group limit, else unlimited" — stale relative to the actual, already-correct
implementation (buildLimitScopes/checkAndCount evaluate every configured
scope and return the first violation). Amended §2 and §7 to state both
scopes are enforced together and the tighter bound wins.

I4 (manifest hedge): .traefik.yml's groups.default was a bare "{}",
the shape empirically reproduced as a decode failure outside this
repo's own (necessarily simplified) yaegi-check harness. Changed to
"providers: []" — same zero semantics, no longer a bare empty object.
manifest_test.go's JSON mirror and drift-check token updated to match;
make yaegi-check still passes (its YAML-subset parser already handles
an inline "[]" under a nested map key).

M9 (pricing provenance): pricing.go's built-in table comment replaced
"verify before release" with a real source note (each provider's own
published list prices, recorded 2026-08, approximate). README's limits
section gained the matching caveat and points to the "pricing" config
override for billing-grade accuracy.

Doc nits: README's Kubernetes section now says the example manifest
applies cleanly once Traefik's CRDs (Middleware, IngressRoute) are
installed, rather than claiming kubectl apply -f works unconditionally.
README's Security notes section no longer claims "no auth-event logging
at all" (see the paired fix: commit) — it now describes what is logged
and that key material never appears.

This commit and its sibling fix: commit are the final wave closing out
the whole-branch review findings for docs/superpowers/sdd/2026-08-20-
traefik-llmgateway.

Gates: go test ./... -count=1 (-race) green, gofmt -l . empty, go vet
clean, make yaegi-check OK, golangci-lint 0 issues.

pricing.go

@@ -10,8 +10,10 @@ import (
 const tokensPerMillion = 1_000_000
 
 // builtinPricing is the built-in per-model price table, in USD per 1M
-// tokens (InputPerM, OutputPerM). Source: public list prices, 2026-08;
-// verify before release.
+// tokens (InputPerM, OutputPerM). Source: each provider's own published
+// list prices, as recorded 2026-08. Approximate: providers change prices
+// without notice, and this table is not refreshed automatically — set the
+// middleware config's "pricing" overrides for billing-grade accuracy.
 var builtinPricing = map[string]ModelPricing{
 	"gpt-5":             {InputPerM: 1.25, OutputPerM: 10},
 	"gpt-5-mini":        {InputPerM: 0.25, OutputPerM: 2},