feat(pi): 新增 Pi (oh-my-pi) harness 支持（关 #44，对齐上游 v6.0.0）

issue #44：Pi 像 opencode 一样开放，skill:// 能直接用，已验证可手动使用，
希望原生支持。上游 obra/superpowers 在 v6.0.0 已用扩展模型原生集成 Pi，
本提交把同样的集成方式落到本 fork。

Pi 走扩展模型，通过 package.json 的 pi 字段声明，直接指向仓库现有
skills/，不复制 skill、无运行时依赖。扩展内容中立——读取 fork 现有的
中文 using-superpowers/SKILL.md 自动注入，故扩展代码逐字节照搬上游。

- .pi/extensions/superpowers.ts：注册 resources_discover / session_start /
  session_compact / agent_end / context 生命周期钩子，在会话注入
  using-superpowers bootstrap + Pi 工具映射（带去重标记、插在 compaction
  summary 之后）
- package.json：加 pi.skills=["./skills"] + pi.extensions + pi-package
  keyword；.pi/extensions/ 加入 files（npm 发布需含扩展）
- skills/using-superpowers/references/pi-tools.md：Pi 工具映射参考
- docs/README.pi.md：中文安装/原理/工具映射/验证指南；README.md 工具列表加链接
- tests/pi/：上游扩展行为测试（适配 fork：name=superpowers-zh）+ 运行包装

验证：bash tests/pi/run-tests.sh 6/6 通过 exit 0（校验 pi 包配置、生命周期
钩子无 pre-compaction 注入、resources_discover 贡献 skills 目录、session_start
注入 You-have-superpowers + Pi-tool-mapping、pi-tools 参考存在）；package.json
合法；scripts/audit.sh 静态 0 FAIL；README→docs/README.pi.md 链接可解析。

注：扩展是 TS（仅 import type，运行时无类型依赖），Node 22.6–23.5 需
--experimental-strip-types（run-tests.sh 已带），23.6+ 默认支持。
Pi 内实际 skill 触发需在 Pi 内验证（与本 fork 其它 harness 同样限制）。

.pi/extensions/superpowers.ts

@@ -0,0 +1,121 @@
+import { readFileSync } from "node:fs";
+import { dirname, resolve } from "node:path";
+import { fileURLToPath } from "node:url";
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
+
+const EXTREMELY_IMPORTANT_MARKER = "<EXTREMELY_IMPORTANT>";
+const BOOTSTRAP_MARKER = "superpowers:using-superpowers bootstrap for pi";
+
+const extensionDir = dirname(fileURLToPath(import.meta.url));
+const packageRoot = resolve(extensionDir, "../..");
+const skillsDir = resolve(packageRoot, "skills");
+const bootstrapSkillPath = resolve(skillsDir, "using-superpowers", "SKILL.md");
+
+let cachedBootstrap: string | null | undefined;
+
+export default function superpowersPiExtension(pi: ExtensionAPI) {
+	let injectBootstrap = true;
+
+	pi.on("resources_discover", async () => ({
+		skillPaths: [skillsDir],
+	}));
+
+	pi.on("session_start", async () => {
+		injectBootstrap = true;
+	});
+
+	pi.on("session_compact", async () => {
+		injectBootstrap = true;
+	});
+
+	pi.on("agent_end", async () => {
+		injectBootstrap = false;
+	});
+
+	pi.on("context", async (event) => {
+		if (!injectBootstrap) return;
+		if (event.messages.some(messageContainsBootstrap)) return;
+
+		const bootstrap = getBootstrapContent();
+		if (!bootstrap) return;
+
+		const bootstrapMessage = {
+			role: "user" as const,
+			content: [{ type: "text" as const, text: bootstrap }],
+			timestamp: Date.now(),
+		};
+
+		const insertAt = firstNonCompactionSummaryIndex(event.messages);
+		return {
+			messages: [
+				...event.messages.slice(0, insertAt),
+				bootstrapMessage,
+				...event.messages.slice(insertAt),
+			],
+		};
+	});
+}
+
+function getBootstrapContent(): string | null {
+	if (cachedBootstrap !== undefined) return cachedBootstrap;
+
+	try {
+		const skillContent = readFileSync(bootstrapSkillPath, "utf8");
+		const body = stripFrontmatter(skillContent);
+		cachedBootstrap = `${EXTREMELY_IMPORTANT_MARKER}
+${BOOTSTRAP_MARKER}
+
+You have superpowers.
+
+The using-superpowers skill content is included below and is already loaded for this Pi session. Follow it now. Do not try to load using-superpowers again.
+
+${body}
+
+${piToolMapping()}
+</EXTREMELY_IMPORTANT>`;
+		return cachedBootstrap;
+	} catch {
+		cachedBootstrap = null;
+		return null;
+	}
+}
+
+function stripFrontmatter(content: string): string {
+	const match = content.match(/^---\n[\s\S]*?\n---\n([\s\S]*)$/);
+	return (match ? match[1] : content).trim();
+}
+
+function piToolMapping(): string {
+	return `## Pi tool mapping
+
+Pi has native skills but does not expose Claude Code's \`Skill\` tool. When a Superpowers instruction says to invoke a skill, use Pi's native skill system instead: load the relevant \`SKILL.md\` with \`read\` when the skill applies, or let a human invoke \`/skill:name\` explicitly.
+
+Pi's built-in coding tools are lowercase: \`read\`, \`write\`, \`edit\`, \`bash\`, plus optional \`grep\`, \`find\`, and \`ls\`. Use those for the corresponding actions: read a file, create or edit files, run shell commands, search file contents, find files by name, and list directories.
+
+Pi does not ship a standard subagent tool. If a subagent tool such as \`subagent\` from \`pi-subagents\` is available, use it for Superpowers subagent workflows. If no subagent tool is available, do the work in this session or explain the missing capability instead of inventing \`Task\` calls.
+
+Pi does not ship a standard task-list tool. If an installed todo/task tool is available, use it. Otherwise track work in plan files or a repo-local \`TODO.md\` when task tracking is needed. Treat older \`TodoWrite\` references as this task-tracking action.`;
+}
+
+function messageContainsBootstrap(message: unknown): boolean {
+	const content = (message as { content?: unknown }).content;
+	if (typeof content === "string") return content.includes(BOOTSTRAP_MARKER);
+	if (!Array.isArray(content)) return false;
+	return content.some((part) => {
+		return (
+			part &&
+			typeof part === "object" &&
+			(part as { type?: unknown }).type === "text" &&
+			typeof (part as { text?: unknown }).text === "string" &&
+			(part as { text: string }).text.includes(BOOTSTRAP_MARKER)
+		);
+	});
+}
+
+function firstNonCompactionSummaryIndex(messages: unknown[]): number {
+	let index = 0;
+	while ((messages[index] as { role?: unknown } | undefined)?.role === "compactionSummary") {
+		index += 1;
+	}
+	return index;
+}