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-categories.ts

@@ -31,3 +31,9 @@ export const CATEGORY_PROMPT_APPENDS: Record<string, string> = buildCategoryReco
 export const CATEGORY_DESCRIPTIONS: Record<string, string> = buildCategoryRecord(
   (definition) => definition.description
 )
+
+export const CATEGORY_PROMPT_APPEND_RESOLVERS: Record<string, (model: string | undefined) => string> = Object.fromEntries(
+  BUILTIN_CATEGORIES
+    .filter((definition) => definition.resolvePromptAppend !== undefined)
+    .map((definition) => [definition.name, definition.resolvePromptAppend!]),
+)