feat(agents): add gpt-5.5 native deep category prompt

Hephaestus 5.5 was rewritten as an outcome-first delegation contract in c3fabaaf. The deep category (spawned as sisyphus-junior under gpt-5.5) now receives a matching prose-driven category context lifted from drafts/gpt-5-5/deep.md instead of the legacy gpt-5.4-era threat-frame version.

Selection happens via a new model-aware resolvePromptAppend hook on BuiltinCategoryDefinition. When the resolved category model is gpt-5.5 the new DEEP_CATEGORY_PROMPT_APPEND_GPT_5_5 is used; older models keep the legacy DEEP_CATEGORY_PROMPT_APPEND. User prompt_append remains preserved on top of either base.

src/tools/delegate-task/builtin-category-definition.ts

@@ -5,4 +5,5 @@ export type BuiltinCategoryDefinition = {
   config: CategoryConfig
   description: string
   promptAppend: string
+  resolvePromptAppend?: (model: string | undefined) => string
 }