🛡️ Sentinel: [Security Enhancement] Add essential security headers to Express servers

[tody-agent]

🚨 Severity: MEDIUM (Security Enhancement)
💡 Vulnerability: Express servers src/dashboard.ts and src/browse-server.ts did not enforce basic security headers, and src/browse-server.ts leaked the underlying server framework version via the x-powered-by header.
🎯 Impact: Without proper security headers, applications are more susceptible to MIME-type sniffing, Clickjacking, and Cross-Site Scripting (XSS) attacks. Leaking the x-powered-by header aids reconnaissance for attackers targeting specific Express.js vulnerabilities.
🔧 Fix:

• Added a custom middleware to src/dashboard.ts and src/browse-server.ts to inject standard security headers (X-Content-Type-Options, X-Frame-Options, X-XSS-Protection).
• Disabled x-powered-by in src/browse-server.ts.
• Logged the findings in .jules/sentinel.md.
  ✅ Verification: Run node scripts/gate-1-syntax.js and node scripts/gate-6-smoke-test.js locally to verify changes, and manually inspect HTTP response headers to ensure the X-Content-Type-Options, X-Frame-Options, and X-XSS-Protection headers are present and x-powered-by is absent.

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PR created automatically by Jules for task 1462523197618334156 started by @tody-agent

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