🛡️ Sentinel: [CRITICAL] Fix Command Injection in browser URL handler

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: The OS-specific URL handler in internal/browser/browser.go used exec.Command directly with the user-supplied URL without any validation, opening up a command injection or SSRF vulnerability if arbitrary or local file URLs were passed.
🎯 Impact: An attacker could craft a malicious URL to execute commands on the host operating system or access local files.
🔧 Fix: Introduced URL scheme validation using net/url.Parse to ensure only http and https schemes are allowed before passing to OS-specific open commands. Also renamed url to rawurl to avoid shadowing the package.
✅ Verification: Verified by running unit tests (go test ./... -short) and ensuring gosec ./... no longer reports an exploitable path, and go vet ./internal/browser/... passes.

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PR created automatically by Jules for task 18337675449359256288 started by @rschumann

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