docs: update hephaestus default model references from gpt-5.3-codex to gpt-5.4

Updated across README (all locales), docs/guide/, docs/reference/,
docs/examples/, AGENTS.md files, and test expectations/snapshots.

The deep category and multimodal-looker still use gpt-5.3-codex as
those are separate from the hephaestus agent.

src/shared/agent-variant.test.ts

@@ -113,9 +113,9 @@ describe("resolveVariantForModel", () => {
   })
 
   test("returns correct variant for openai provider (hephaestus agent)", () => {
-    // #given hephaestus has openai/gpt-5.3-codex with variant "medium" in its chain
+    // #given hephaestus has openai/gpt-5.4 with variant "medium" in its chain
     const config = {} as OhMyOpenCodeConfig
-    const model = { providerID: "openai", modelID: "gpt-5.3-codex" }
+    const model = { providerID: "openai", modelID: "gpt-5.4" }
 
     // #when
     const variant = resolveVariantForModel(config, "hephaestus", model)