feat: implement skill tool for loading and discovering skills

- Add skill tool types: SkillArgs, SkillInfo, SkillLoadOptions interfaces
- Implement createSkillTool() factory function with configurable discovery options
- Add parseSkillInfo() helper to convert LoadedSkill to user-facing SkillInfo format
- Add formatSkillsXml() helper to generate available skills XML for tool description
- Support opencodeOnly option to filter Claude Code paths from discovery
- Tool loads and parses skill frontmatter, returns skill content with base directory
- Export skill tool singleton instance for default usage

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/skill/constants.ts

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+export const TOOL_NAME = "skill" as const
+
+export const TOOL_DESCRIPTION_NO_SKILLS = "Load a skill to get detailed instructions for a specific task. No skills are currently available."
+
+export const TOOL_DESCRIPTION_PREFIX = `Load a skill to get detailed instructions for a specific task.
+
+Skills provide specialized knowledge and step-by-step guidance.
+Use this when a task matches an available skill's description.`

src/tools/skill/index.ts

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+export * from "./constants"
+export * from "./types"
+export { skill, createSkillTool } from "./tools"

src/tools/skill/tools.ts

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+import { dirname } from "node:path"
+import { readFileSync } from "node:fs"
+import { tool, type ToolDefinition } from "@opencode-ai/plugin"
+import { TOOL_DESCRIPTION_NO_SKILLS, TOOL_DESCRIPTION_PREFIX } from "./constants"
+import type { SkillArgs, SkillInfo, SkillLoadOptions } from "./types"
+import { discoverSkills, getSkillByName, type LoadedSkill } from "../../features/opencode-skill-loader"
+import { parseFrontmatter } from "../../shared/frontmatter"
+
+function loadedSkillToInfo(skill: LoadedSkill): SkillInfo {
+  return {
+    name: skill.name,
+    description: skill.definition.description || "",
+    location: skill.path,
+    scope: skill.scope,
+    license: skill.license,
+    compatibility: skill.compatibility,
+    metadata: skill.metadata,
+    allowedTools: skill.allowedTools,
+  }
+}
+
+function formatSkillsXml(skills: SkillInfo[]): string {
+  if (skills.length === 0) return ""
+
+  const skillsXml = skills.map(skill => {
+    const lines = [
+      "  <skill>",
+      `    <name>${skill.name}</name>`,
+      `    <description>${skill.description}</description>`,
+    ]
+    if (skill.compatibility) {
+      lines.push(`    <compatibility>${skill.compatibility}</compatibility>`)
+    }
+    lines.push("  </skill>")
+    return lines.join("\n")
+  }).join("\n")
+
+  return `\n\n<available_skills>\n${skillsXml}\n</available_skills>`
+}
+
+export function createSkillTool(options: SkillLoadOptions = {}): ToolDefinition {
+  const skills = discoverSkills({ includeClaudeCodePaths: !options.opencodeOnly })
+  const skillInfos = skills.map(loadedSkillToInfo)
+
+  const description = skillInfos.length === 0
+    ? TOOL_DESCRIPTION_NO_SKILLS
+    : TOOL_DESCRIPTION_PREFIX + formatSkillsXml(skillInfos)
+
+  return tool({
+    description,
+    args: {
+      name: tool.schema.string().describe("The skill identifier from available_skills (e.g., 'code-review')"),
+    },
+    async execute(args: SkillArgs) {
+      const skill = getSkillByName(args.name, { includeClaudeCodePaths: !options.opencodeOnly })
+
+      if (!skill) {
+        const available = skills.map(s => s.name).join(", ")
+        throw new Error(`Skill "${args.name}" not found. Available skills: ${available || "none"}`)
+      }
+
+      const content = readFileSync(skill.path, "utf-8")
+      const { body } = parseFrontmatter(content)
+      const dir = dirname(skill.path)
+
+      const output = [
+        `## Skill: ${skill.name}`,
+        "",
+        `**Base directory**: ${dir}`,
+        "",
+        body.trim(),
+      ].join("\n")
+
+      return output
+    },
+  })
+}
+
+export const skill = createSkillTool()

src/tools/skill/types.ts

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+export interface SkillArgs {
+  name: string
+}
+
+export interface SkillInfo {
+  name: string
+  description: string
+  location: string
+  scope: "opencode-project" | "project" | "opencode" | "user"
+  license?: string
+  compatibility?: string
+  metadata?: Record<string, string>
+  allowedTools?: string[]
+}
+
+export interface SkillLoadOptions {
+  /** When true, only load from OpenCode paths (.opencode/skill/, ~/.config/opencode/skill/) */
+  opencodeOnly?: boolean
+}