🛡️ Sentinel: Enforce Unix socket peer verification

[minibota]

🔐 Issue: cc-agent exec blindly connected to the Unix socket specified by --socket-path (or fallback). If the socket path was in a shared directory (like /tmp, which is used as a fallback or in User Mode), a local attacker could pre-create the socket and intercept execution reports containing sensitive job output (stdout/stderr).

🧠 Risk: Information Disclosure (High). An unprivileged local attacker could capture output from cron jobs running as other users.

🛠️ Fix:

• Implemented verifySocketPeer using syscall.GetsockoptUcred (on Linux) to validate the peer's UID.
• The connection is now rejected unless the peer UID matches the current user OR is root (allowing connection to System Mode daemon).
• Added var getCurrentUid to allow mocking in tests.

🧪 Tests:

• Added cmd/security_peer_linux_test.go:
  • Verified successful connection when UIDs match.
  • Verified connection rejection when UIDs mismatch (mocked).
• Ran go test ./cmd (All Pass).

📎 Notes: This fix focuses on the client side (exec command). The daemon side already sets 0660 permissions, but this adds defense-in-depth against pre-creation attacks in shared directories.

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PR created automatically by Jules for task 17519485796025406417 started by @minibota

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