feat(hephaestus): add generic GPT prompt fallback with model-specific routing

Split monolithic hephaestus.ts into directory with model-specific prompt
variants (gpt-5-4.ts, gpt-5-3-codex.ts, gpt.ts) mirroring the
sisyphus-junior pattern. Generic gpt.ts uses pre-codex-tuning prompt as
fallback for non-specific GPT models.

Also adds isGpt5_4Model and isGpt5_3CodexModel helpers to types.ts.

src/agents/hephaestus/index.ts

@@ -0,0 +1,8 @@
+export {
+  createHephaestusAgent,
+  getHephaestusPrompt,
+  getHephaestusPromptSource,
+  hephaestusPromptMetadata,
+} from "./agent";
+
+export type { HephaestusContext, HephaestusPromptSource } from "./agent";