fix(security): add archive extraction containment validation

Validate tar and zip entries before extraction to prevent path traversal:
- Reject absolute paths in archives
- Reject .. traversal paths
- Reject symlinks pointing outside extraction dir
- New archive-entry-validator module with comprehensive tests

Addresses: security audit finding for unsafe archive extraction.

src/shared/binary-downloader.ts

@@ -1,6 +1,7 @@
 import { chmodSync, existsSync, mkdirSync, unlinkSync } from "node:fs";
 import * as path from "node:path";
 import { spawn } from "bun";
+import { validateArchiveEntries, type ArchiveEntry } from "./archive-entry-validator";
 import { extractZip } from "./zip-extractor";
 
 export function getCachedBinaryPath(cacheDir: string, binaryName: string): string | null {
@@ -29,6 +30,9 @@ export async function extractTarGz(
   destDir: string,
   options?: { args?: string[]; cwd?: string }
 ): Promise<void> {
+  const entries = await listTarEntries(archivePath, options?.cwd)
+  validateArchiveEntries(entries, destDir)
+
   const args = options?.args ?? ["tar", "-xzf", archivePath, "-C", destDir];
   const proc = spawn(args, {
     cwd: options?.cwd,
@@ -58,3 +62,54 @@ export function ensureExecutable(binaryPath: string): void {
     chmodSync(binaryPath, 0o755);
   }
 }
+
+function parseTarEntry(line: string): ArchiveEntry | null {
+  const match = line.match(/^([^\s])\S*\s+\d+\s+\S+\s+\S+\s+\d+\s+\w+\s+\d+\s+(?:\d{2}:\d{2}|\d{4})\s+(.*)$/)
+  if (!match) {
+    return null
+  }
+
+  const [, rawType, rawEntryPath] = match
+  if (rawType === "l") {
+    const arrowIndex = rawEntryPath.lastIndexOf(" -> ")
+    if (arrowIndex === -1) {
+      return { path: rawEntryPath, type: "symlink" }
+    }
+
+    return {
+      path: rawEntryPath.slice(0, arrowIndex),
+      type: "symlink",
+      linkPath: rawEntryPath.slice(arrowIndex + 4),
+    }
+  }
+
+  return {
+    path: rawEntryPath,
+    type: rawType === "d" ? "directory" : "file",
+  }
+}
+
+async function listTarEntries(archivePath: string, cwd?: string): Promise<ArchiveEntry[]> {
+  const proc = spawn(["tar", "-tvzf", archivePath], {
+    cwd,
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+
+  const [exitCode, stdout, stderr] = await Promise.all([
+    proc.exited,
+    new Response(proc.stdout).text(),
+    new Response(proc.stderr).text(),
+  ])
+
+  if (exitCode !== 0) {
+    throw new Error(`tar entry listing failed (exit ${exitCode}): ${stderr}`)
+  }
+
+  return stdout
+    .split(/\r?\n/)
+    .map(line => line.trim())
+    .filter(Boolean)
+    .map(line => parseTarEntry(line))
+    .filter((entry): entry is ArchiveEntry => entry !== null)
+}