fix(security): Add integrity verification before chmod +x in btrace-perfetto skill

Add validation to verify downloaded trace_processor file is a valid executable
before making it executable. This prevents potential execution of malicious or
corrupted downloads.

Changes:
- Verify file exists and has non-zero size
- Check file type to confirm it's an executable
- Remove file and exit with error if validation fails
- Only chmod +x after successful verification

Fixes: https://linear.app/getsentry/issue/EME-1060
Parent ticket: https://linear.app/getsentry/issue/VULN-1513

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: fix-it-felix-sentry[bot]

.claude/skills/btrace-perfetto/SKILL.md

@@ -22,7 +22,18 @@ Before starting, verify:
    ```
 3. **Perfetto trace_processor**: Check if `/tmp/trace_processor` exists. If not, download it:
    ```bash
-   curl -sL "https://get.perfetto.dev/trace_processor" -o /tmp/trace_processor && chmod +x /tmp/trace_processor
+   # Download trace_processor
+   curl -sL "https://get.perfetto.dev/trace_processor" -o /tmp/trace_processor
+
+   # Verify the file is a valid executable (check file type and size)
+   if [[ ! -s /tmp/trace_processor ]] || ! file /tmp/trace_processor | grep -q "executable"; then
+     echo "Error: Downloaded file is not a valid executable"
+     rm -f /tmp/trace_processor
+     exit 1
+   fi
+
+   # Make executable only after verification
+   chmod +x /tmp/trace_processor
    ```
 4. **Device ABI**: Run `adb shell getprop ro.product.cpu.abi` — btrace only supports arm64-v8a and armeabi-v7a (no x86/x86_64)