fix: propagate project skills to background task sessions (#2687)

prompt-builder.ts now includes project-level skills from
.opencode/skills/ when building delegated session prompts.

5007 tests pass, tsc clean.

Closes #2687

src/tools/delegate-task/prompt-builder.ts

@@ -1,4 +1,5 @@
 import type { BuildSystemContentInput } from "./types"
+import type { AvailableSkill } from "../../agents/dynamic-agent-prompt-builder"
 import { buildPlanAgentSystemPrepend, isPlanAgent } from "./constants"
 import { buildSystemContentWithTokenLimit } from "./token-limiter"
 
@@ -21,6 +22,22 @@ ${TDD_LINE}`
   return PLAN_AGENT_PROMPT_BASE
 }
 
+function buildAvailableSkillsSection(skills: AvailableSkill[]): string {
+  if (skills.length === 0) {
+    return ""
+  }
+
+  const rows = skills
+    .map((s) => `- \`${s.name}\`: ${s.description || s.name}`)
+    .join("\n")
+
+  return `<available_skills>
+Skills provide specialized instructions. Load via load_skills parameter when delegating tasks.
+
+${rows}
+</available_skills>`
+}
+
 function usesFreeOrLocalModel(model: { providerID: string; modelID: string; variant?: string } | undefined): boolean {
   if (!model) {
     return false
@@ -51,10 +68,20 @@ export function buildSystemContent(input: BuildSystemContentInput): string | und
     availableSkills,
   } = input
 
-  const planAgentPrepend = isPlanAgent(agentName)
+  const isPlan = isPlanAgent(agentName)
+  const planAgentPrepend = isPlan
     ? buildPlanAgentSystemPrepend(availableCategories, availableSkills)
     : ""
 
+  const skillsSection = !isPlan
+    ? buildAvailableSkillsSection(availableSkills ?? [])
+    : ""
+
+  const baseAgentsContext = agentsContext ?? planAgentPrepend
+  const effectiveAgentsContext = !isPlan && skillsSection
+    ? [baseAgentsContext, skillsSection].filter(Boolean).join("\n\n")
+    : baseAgentsContext
+
   const effectiveMaxPromptTokens = maxPromptTokens
     ?? (usesFreeOrLocalModel(model) ? FREE_OR_LOCAL_PROMPT_TOKEN_LIMIT : undefined)
 
@@ -63,7 +90,7 @@ export function buildSystemContent(input: BuildSystemContentInput): string | und
       skillContent,
       skillContents,
       categoryPromptAppend,
-      agentsContext: agentsContext ?? planAgentPrepend,
+      agentsContext: effectiveAgentsContext,
       planAgentPrepend,
     },
     effectiveMaxPromptTokens