fix: use npm install for Claude Code CLI on musl/Alpine systems

The native binary (install.sh) requires posix_getdents which is only
available in Alpine edge (musl 1.2.5-r21+), not in any stable Alpine
release. Detect musl upfront via ldd and skip straight to npm install,
avoiding 15+ seconds of wasted retries on deterministic failures.

On glibc systems the native install path is unchanged with npm as a
fallback if it fails.

src/entrypoints/run.ts

@@ -8,7 +8,7 @@
 
 import * as core from "@actions/core";
 import { dirname } from "path";
-import { spawn } from "child_process";
+import { spawn, spawnSync } from "child_process";
 import { appendFile } from "fs/promises";
 import { existsSync, readFileSync } from "fs";
 import { setupGitHubToken, WorkflowValidationSkipError } from "../github/token";
@@ -57,15 +57,53 @@ async function installClaudeCode(): Promise<void> {
   const claudeCodeVersion = "2.1.61";
   console.log(`Installing Claude Code v${claudeCodeVersion}...`);
 
+  // On musl/Alpine systems, the native binary may require a newer musl than
+  // available (e.g. posix_getdents added in musl 1.2.5-r21, only in Alpine edge).
+  // Skip straight to npm install to avoid wasting time on deterministic failures.
+  if (isMusl()) {
+    console.log(
+      "Detected musl libc, using npm install (native binary may be incompatible)...",
+    );
+    await npmInstall(claudeCodeVersion);
+    return;
+  }
+
+  // On glibc systems, try the native binary install with retry for transient failures.
+  const nativeInstallSuccess = await tryNativeInstall(claudeCodeVersion);
+  if (nativeInstallSuccess) return;
+
+  // Fall back to npm if native install fails on glibc too (shouldn't happen normally).
+  console.log("Native install failed, falling back to npm install...");
+  await npmInstall(claudeCodeVersion);
+}
+
+/**
+ * Detect if the system uses musl libc (Alpine, etc.).
+ */
+function isMusl(): boolean {
+  try {
+    const output = spawnSync("ldd", ["--version"], { encoding: "utf-8" });
+    // musl's ldd prints "musl libc" to stderr
+    return (output.stderr + output.stdout).toLowerCase().includes("musl");
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Try installing Claude Code via the native install script.
+ * Returns true on success, false on failure.
+ */
+async function tryNativeInstall(version: string): Promise<boolean> {
   for (let attempt = 1; attempt <= 3; attempt++) {
-    console.log(`Installation attempt ${attempt}...`);
+    console.log(`Native installation attempt ${attempt}...`);
     try {
       await new Promise<void>((resolve, reject) => {
         const child = spawn(
           "bash",
           [
             "-c",
-            `curl -fsSL https://claude.ai/install.sh | bash -s -- ${claudeCodeVersion}`,
+            `curl -fsSL https://claude.ai/install.sh | bash -s -- ${version}`,
           ],
           { stdio: "inherit" },
         );
@@ -75,25 +113,59 @@ async function installClaudeCode(): Promise<void> {
         });
         child.on("error", reject);
       });
-      console.log("Claude Code installed successfully");
-      // Add to PATH
+      console.log("Claude Code installed successfully (native)");
       const homeBin = `${process.env.HOME}/.local/bin`;
       const githubPath = process.env.GITHUB_PATH;
       if (githubPath) {
         await appendFile(githubPath, `${homeBin}\n`);
       }
       process.env.PATH = `${homeBin}:${process.env.PATH}`;
-      return;
-    } catch (error) {
+      return true;
+    } catch {
       if (attempt === 3) {
-        throw new Error(
-          `Failed to install Claude Code after 3 attempts: ${error}`,
-        );
+        return false;
       }
       console.log("Installation failed, retrying...");
       await new Promise((resolve) => setTimeout(resolve, 5000));
     }
   }
+  return false;
+}
+
+/**
+ * Install Claude Code via npm as a fallback for musl/Alpine systems where
+ * the native binary may be incompatible (e.g. posix_getdents symbol missing).
+ */
+async function npmInstall(version: string): Promise<void> {
+  try {
+    await new Promise<void>((resolve, reject) => {
+      const child = spawn(
+        "npm",
+        ["install", "-g", `@anthropic-ai/claude-code@${version}`],
+        { stdio: "inherit" },
+      );
+      child.on("close", (code) => {
+        if (code === 0) resolve();
+        else reject(new Error(`npm install failed with exit code ${code}`));
+      });
+      child.on("error", reject);
+    });
+    console.log("Claude Code installed successfully (npm)");
+    // Resolve the npm global bin directory and ensure it's in PATH.
+    // Use `npm prefix -g` + /bin (npm bin -g is deprecated since npm v9).
+    const npmPrefix = spawnSync("npm", ["prefix", "-g"], { encoding: "utf-8" });
+    const npmBin =
+      npmPrefix.status === 0 ? `${npmPrefix.stdout.trim()}/bin` : "";
+    if (npmBin) {
+      const githubPath = process.env.GITHUB_PATH;
+      if (githubPath) {
+        await appendFile(githubPath, `${npmBin}\n`);
+      }
+      process.env.PATH = `${npmBin}:${process.env.PATH}`;
+    }
+  } catch (error) {
+    throw new Error(`Failed to install Claude Code via npm: ${error}`);
+  }
 }
 
 /**