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.

mcp_a2a.go

@@ -185,13 +185,14 @@ func (g *Gateway) handleTargetProxy(w http.ResponseWriter, r *http.Request, u *u
 
 // validateTargetURLs checks every configured MCP-server and agent entry at
 // construction: the map value must be non-nil, the name must pass
-// validateConfigName (same character set and reserved-name rules as a
-// provider name — an MCP server or agent is routed at "/mcp/{name}/*" or
-// "/a2a/{name}/*", the same shadowing risk applies), and the target URL
-// must parse (url.Parse) and use an http or https scheme. A nil value,
-// invalid name, or malformed/non-HTTP target is rejected once, here, at
-// construction — a constructor error, not a nil-pointer panic or a 502 the
-// first time some caller happens to address it.
+// validateConfigName (the same character-set, dot-only, and reserved-name
+// rules as a provider name — an MCP server or agent is routed at
+// "/mcp/{name}/*" or "/a2a/{name}/*", so the same shadowing risk and the
+// same "." / ".." directory-traversal-segment risk both apply), and the
+// target URL must parse (url.Parse) and use an http or https scheme. A nil
+// value, invalid name, or malformed/non-HTTP target is rejected once, here,
+// at construction — a constructor error, not a nil-pointer panic or a 502
+// the first time some caller happens to address it.
 func validateTargetURLs(cfg *Config) error {
 	for name, tc := range cfg.MCPServers {
 		if tc == nil {