fix(skill): unify skill resolution to support user custom skills

sisyphus_task was only loading builtin skills via resolveMultipleSkills().
Now uses resolveMultipleSkillsAsync() which merges discoverSkills() + builtin skills.

- Add getAllSkills(), extractSkillTemplate(), resolveMultipleSkillsAsync()
- Update sisyphus_task to use async skill resolution
- Refactor skill tool to reuse unified getAllSkills()
- Add async skill resolution tests

src/features/opencode-skill-loader/skill-content.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "bun:test"
-import { resolveSkillContent, resolveMultipleSkills } from "./skill-content"
+import { resolveSkillContent, resolveMultipleSkills, resolveSkillContentAsync, resolveMultipleSkillsAsync } from "./skill-content"
 
 describe("resolveSkillContent", () => {
 	it("should return template for existing skill", () => {
@@ -109,3 +109,87 @@ describe("resolveMultipleSkills", () => {
 		expect(result.resolved.size).toBe(2)
 	})
 })
+
+describe("resolveSkillContentAsync", () => {
+	it("should return template for builtin skill", async () => {
+		// #given: builtin skill 'frontend-ui-ux'
+		// #when: resolving content async
+		const result = await resolveSkillContentAsync("frontend-ui-ux")
+
+		// #then: returns template string
+		expect(result).not.toBeNull()
+		expect(typeof result).toBe("string")
+		expect(result).toContain("Role: Designer-Turned-Developer")
+	})
+
+	it("should return null for non-existent skill", async () => {
+		// #given: non-existent skill name
+		// #when: resolving content async
+		const result = await resolveSkillContentAsync("definitely-not-a-skill-12345")
+
+		// #then: returns null
+		expect(result).toBeNull()
+	})
+})
+
+describe("resolveMultipleSkillsAsync", () => {
+	it("should resolve builtin skills", async () => {
+		// #given: builtin skill names
+		const skillNames = ["playwright", "frontend-ui-ux"]
+
+		// #when: resolving multiple skills async
+		const result = await resolveMultipleSkillsAsync(skillNames)
+
+		// #then: all builtin skills resolved
+		expect(result.resolved.size).toBe(2)
+		expect(result.notFound).toEqual([])
+		expect(result.resolved.get("playwright")).toContain("Playwright Browser Automation")
+		expect(result.resolved.get("frontend-ui-ux")).toContain("Designer-Turned-Developer")
+	})
+
+	it("should handle partial success with non-existent skills", async () => {
+		// #given: mix of existing and non-existing skills
+		const skillNames = ["playwright", "nonexistent-skill-12345"]
+
+		// #when: resolving multiple skills async
+		const result = await resolveMultipleSkillsAsync(skillNames)
+
+		// #then: existing skills resolved, non-existing in notFound
+		expect(result.resolved.size).toBe(1)
+		expect(result.notFound).toEqual(["nonexistent-skill-12345"])
+		expect(result.resolved.get("playwright")).toContain("Playwright Browser Automation")
+	})
+
+	it("should support git-master config injection", async () => {
+		// #given: git-master skill with config override
+		const skillNames = ["git-master"]
+		const options = {
+			gitMasterConfig: {
+				commit_footer: false,
+				include_co_authored_by: false,
+			},
+		}
+
+		// #when: resolving with git-master config
+		const result = await resolveMultipleSkillsAsync(skillNames, options)
+
+		// #then: config values injected into template
+		expect(result.resolved.size).toBe(1)
+		expect(result.notFound).toEqual([])
+		const gitMasterContent = result.resolved.get("git-master")
+		expect(gitMasterContent).toContain("commit_footer")
+		expect(gitMasterContent).toContain("DISABLED")
+	})
+
+	it("should handle empty array", async () => {
+		// #given: empty skill names
+		const skillNames: string[] = []
+
+		// #when: resolving multiple skills async
+		const result = await resolveMultipleSkillsAsync(skillNames)
+
+		// #then: empty results
+		expect(result.resolved.size).toBe(0)
+		expect(result.notFound).toEqual([])
+	})
+})