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.test.ts

@@ -0,0 +1,125 @@
+declare const require: (name: string) => unknown
+const { describe, test, expect } = require("bun:test") as {
+  describe: (name: string, fn: () => void) => void
+  test: (name: string, fn: () => void) => void
+  expect: (value: unknown) => {
+    toBe: (expected: unknown) => void
+    toContain: (expected: string) => void
+    toBeUndefined: () => void
+    toBeDefined: () => void
+    not: {
+      toContain: (expected: string) => void
+      toBeUndefined: () => void
+    }
+  }
+}
+
+import { buildSystemContent } from "./prompt-builder"
+import type { AvailableSkill, AvailableCategory } from "../../agents/dynamic-agent-prompt-builder"
+
+describe("prompt-builder", () => {
+  describe("buildSystemContent", () => {
+    describe("#given non-plan agent with availableSkills", () => {
+      test("#when availableSkills contains project-level skills #then system content includes available_skills section", () => {
+        // given
+        const availableSkills: AvailableSkill[] = [
+          { name: "git-master", description: "Git workflow automation", location: "plugin" },
+          { name: "my-project-skill", description: "Project-specific deployment", location: "project" },
+        ]
+        const availableCategories: AvailableCategory[] = [
+          { name: "quick", description: "Trivial tasks", model: "openai/gpt-5.4-mini" },
+        ]
+
+        // when
+        const result = buildSystemContent({
+          agentName: "sisyphus-junior",
+          availableSkills,
+          availableCategories,
+        })
+
+        // then
+        expect(result).toBeDefined()
+        expect(result).toContain("my-project-skill")
+        expect(result).toContain("git-master")
+      })
+
+      test("#when agent is explore #then system content includes available_skills section", () => {
+        // given
+        const availableSkills: AvailableSkill[] = [
+          { name: "code-review", description: "Review code quality", location: "project" },
+        ]
+
+        // when
+        const result = buildSystemContent({
+          agentName: "explore",
+          availableSkills,
+        })
+
+        // then
+        expect(result).toBeDefined()
+        expect(result).toContain("code-review")
+      })
+
+      test("#when availableSkills is empty #then system content does not include available_skills section", () => {
+        // given
+        const availableSkills: AvailableSkill[] = []
+
+        // when
+        const result = buildSystemContent({
+          agentName: "sisyphus-junior",
+          availableSkills,
+          categoryPromptAppend: "some category context",
+        })
+
+        // then
+        expect(result).toBeDefined()
+        expect(result).not.toContain("available_skills")
+      })
+    })
+
+    describe("#given plan agent with availableSkills", () => {
+      test("#when availableSkills provided #then system content includes plan agent prepend with skills", () => {
+        // given
+        const availableSkills: AvailableSkill[] = [
+          { name: "git-master", description: "Git workflow automation", location: "plugin" },
+        ]
+        const availableCategories: AvailableCategory[] = [
+          { name: "quick", description: "Trivial tasks", model: "openai/gpt-5.4-mini" },
+        ]
+
+        // when
+        const result = buildSystemContent({
+          agentName: "plan",
+          availableSkills,
+          availableCategories,
+        })
+
+        // then
+        expect(result).toBeDefined()
+        expect(result).toContain("git-master")
+        expect(result).toContain("AVAILABLE SKILLS")
+      })
+    })
+
+    describe("#given non-plan agent with agentsContext override", () => {
+      test("#when agentsContext is provided #then it takes precedence and skills section is appended", () => {
+        // given
+        const availableSkills: AvailableSkill[] = [
+          { name: "deploy-skill", description: "Deployment automation", location: "project" },
+        ]
+
+        // when
+        const result = buildSystemContent({
+          agentName: "sisyphus-junior",
+          agentsContext: "Custom agent context here",
+          availableSkills,
+        })
+
+        // then
+        expect(result).toBeDefined()
+        expect(result).toContain("Custom agent context here")
+        expect(result).toContain("deploy-skill")
+      })
+    })
+  })
+})