fix: Phase 3 — command injection, unwrap/expect panics, unsigned image acceptance

- VPN key gen: replaced sh -c with format string (command injection) with
  safe stdin piping to wg pubkey
- Secrets manager: replaced .unwrap() on path.parent() with proper error
- Tor proxy: replaced .expect("valid proxy") with continue on error
- Image verifier: added require_signatures flag, strict mode rejects
  unsigned images and missing cosign binary

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/security/src/secrets_manager.rs

@@ -109,7 +109,9 @@ impl SecretsManager {
             .join(app_id)
             .join(format!("{}.secret", secret_id));
 
-        fs::create_dir_all(secret_path.parent().unwrap()).await?;
+        let parent = secret_path.parent()
+            .ok_or_else(|| anyhow::anyhow!("Invalid secret path: no parent directory for {:?}", secret_path))?;
+        fs::create_dir_all(parent).await?;
 
         let encrypted = self
             .encrypt(value.as_bytes())