🛡️ Sentinel: Fix insecure socket creation in shared directories

[minibota]

🔐 Issue: The daemon created the Unix socket file directly in /tmp (e.g., /tmp/cc-agent-user.sock) when XDG_RUNTIME_DIR was missing. This location is shared and sticky, allowing local attackers to pre-create the file to cause Denial of Service (DoS) or exploit race conditions.

🧠 Risk: A local attacker could prevent the agent from starting or potentially intercept connections if race conditions are exploited.

🛠️ Fix:

• Created ensureSocketDir to enforce 0700 permissions and ownership on the socket directory.
• Updated getSocketPath to use a dedicated subdirectory based on Effective UID: /tmp/cc-agent-<euid>/cc-agent.sock.
• Daemon now creates this directory securely before listening.

🧪 Tests:

• Added TestEnsureSocketDir to verify directory creation, permission checks (0700), and rejection of insecure permissions.
• Added TestGetSocketPath_Structure to verify path generation logic.
• Ran all existing tests to ensure no regressions.

📎 Notes: This change is transparent to users but changes the default socket location on systems without XDG_RUNTIME_DIR. exec command automatically resolves the new path.

This PR was created by an automated nightly security task.

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PR created automatically by Jules for task 11022395506798640199 started by @minibota

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