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/category-resolver.ts

@@ -5,6 +5,7 @@ import type { FallbackEntry } from "../../shared/model-requirements"
 import { mergeCategories } from "../../shared/merge-categories"
 import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { resolveCategoryConfig } from "./categories"
+import { CATEGORY_PROMPT_APPEND_RESOLVERS } from "./constants"
 import { parseModelString } from "../../shared/model-string-parser"
 import { CATEGORY_MODEL_REQUIREMENTS } from "../../shared/model-requirements"
 import { normalizeFallbackModels, flattenToFallbackModelStrings } from "../../shared/model-resolver"
@@ -26,6 +27,23 @@ function applyCategoryParams(base: DelegatedModelConfig, config: CategoryConfig)
   return result
 }
 
+function resolveCategoryPromptAppendForModel(
+  categoryName: string,
+  actualModel: string | undefined,
+  staticPromptAppend: string,
+  userPromptAppend: string | undefined,
+): string | undefined {
+  const dynamicResolver = CATEGORY_PROMPT_APPEND_RESOLVERS[categoryName]
+  if (!dynamicResolver) {
+    return staticPromptAppend || undefined
+  }
+  const dynamicBase = dynamicResolver(actualModel)
+  if (!userPromptAppend) {
+    return dynamicBase || undefined
+  }
+  return dynamicBase ? `${dynamicBase}\n\n${userPromptAppend}` : userPromptAppend
+}
+
 export interface CategoryResolutionResult {
   agentToUse: string
   categoryModel: DelegatedModelConfig | undefined
@@ -210,7 +228,12 @@ Available categories: ${allCategoryNames}`,
     const parsedModel = parseModelString(actualModel)
     categoryModel = parsedModel ?? undefined
   }
-  const categoryPromptAppend = resolved.promptAppend || undefined
+  const categoryPromptAppend = resolveCategoryPromptAppendForModel(
+    args.category!,
+    actualModel,
+    resolved.promptAppend,
+    userCategories?.[args.category!]?.prompt_append,
+  )
 
   if (!categoryModel && !actualModel && !isModelResolutionSkipped) {
     const categoryNames = Object.keys(enabledCategories)