🛡️ Sentinel: [CRITICAL] Fix timing attack vulnerability in auth token validation

[google-labs-jules]

🚨 Severity: CRITICAL
💡 Vulnerability: The authentication endpoint used a short-circuit length comparison before running crypto.timingSafeEqual when verifying Bearer tokens.
🎯 Impact: This fast path leaks the length of the expected valid authentication token via timing differences, allowing an attacker to narrow down the possible token variations.
🔧 Fix: Removed the direct length comparison and Buffer.from() conversion. Instead, both the user-provided token and the configured token are hashed using SHA-256 to a fixed length before being compared with crypto.timingSafeEqual.
✅ Verification: Ran bun test ./tests/unit/cli/commands/serve.test.ts and verified the full test suite passes with bun run test:full.

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PR created automatically by Jules for task 10451291637956531233 started by @YoungSx

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