fix(security): add AWS secret key validation and harden path traversal (#2690)

- Add validateAwsSecretKey() function checking 40-char format
- Validate secret key in loadCredsFromConfig() and lightsailRest()
- Add normalize() to canonicalize paths before traversal check
- Harden both uploadFile() and downloadFile() path validation
- Update test fixtures with properly-formatted mock secret keys
- Add test for invalid secret key format rejection

Fixes #2686
Fixes #2687

Agent: security-auditor

Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.20.2",
+  "version": "0.20.3",
   "type": "module",
   "bin": {
     "spawn": "cli.js"