ci: prune openai/gpt-pro from default models-live matrix (#637)

* ci: prune openai/gpt-pro from default models-live matrix

gpt-5.5-pro burns ~$2.40/run ($30/M input, $180/M output) — flagship
reasoning tier with hidden reasoning tokens dominating cost. Multiplied
by every push that touches a resolution-affecting file, the bill is
untenable for a smoke that just verifies set_output works.

Pruned by default; re-enable with INCLUDE_EXPENSIVE=1 or MATRIX_FILTER
when validating the alias on demand.

Also adds a comment-frugality rule to AGENTS.md.

* ci: include list-aliases.ts in models paths-filter

The matrix builder is resolution-affecting from a validation standpoint
— a regression to it (e.g. accidentally pruning all aliases) wouldn't
trigger models-live on its own commit.

test/list-aliases.ts

@@ -18,6 +18,7 @@
  *   node action/test/list-aliases.ts
  *   MATRIX_FILTER=gemini node action/test/list-aliases.ts
  *   INCLUDE_ALL_PASSTHROUGHS=1 node action/test/list-aliases.ts
+ *   INCLUDE_EXPENSIVE=1 node action/test/list-aliases.ts
  */
 import { modelAliases } from "../models.ts";
 
@@ -29,6 +30,10 @@ function agentForSlug(slug: string): "claude" | "opencode" {
 // translation) is alive without re-testing every underlying model.
 const ROUTING_CANARIES = new Set(["openrouter/claude-sonnet", "opencode/claude-sonnet"]);
 
+// pruned by default; opt back in with INCLUDE_EXPENSIVE=1 or MATRIX_FILTER.
+// gpt-5.5-pro burns ~$2.40/run on this fixture — too expensive per-push.
+const EXPENSIVE_ALIASES = new Set(["openai/gpt-pro"]);
+
 function isPrunablePassthrough(alias: (typeof modelAliases)[number]): boolean {
   if (ROUTING_CANARIES.has(alias.slug)) return false;
   if (alias.provider === "openrouter") return true;
@@ -40,10 +45,12 @@ function isPrunablePassthrough(alias: (typeof modelAliases)[number]): boolean {
 
 const filter = process.env.MATRIX_FILTER?.trim() ?? "";
 const includeAllPassthroughs = process.env.INCLUDE_ALL_PASSTHROUGHS === "1";
+const includeExpensive = process.env.INCLUDE_EXPENSIVE === "1" || filter !== "";
 
 const matrix = modelAliases
   .filter((alias) => (filter ? alias.slug.toLowerCase().includes(filter.toLowerCase()) : true))
   .filter((alias) => includeAllPassthroughs || !isPrunablePassthrough(alias))
+  .filter((alias) => includeExpensive || !EXPENSIVE_ALIASES.has(alias.slug))
   .map((alias) => ({
     slug: alias.slug,
     agent: agentForSlug(alias.slug),