refactor: remove dependency on ip CLI tool and abstract network logic

Eliminate the external dependency on the `ip` (iproute2) command-line tool by centralizing network configuration and diagnostics within a new `internal/network` package using the `netlink` library.

Changes:
- Introduced `internal/network` package to handle network interface listing and configuration.
- Replaced `exec.Command("ip", "link")` in `internal/namespace.VerifyIsolation` with `network.ListInterfaces()`.
- Improved `VerifyIsolation` to explicitly ensure only the loopback interface is present in a fresh network namespace.
- Moved interface and routing configuration logic from `internal/wireguard` to `internal/network`.
- Removed unnecessary `os/exec` imports from network-related files.

This change increases the tool's portability by removing the requirement for `iproute2` to be installed in the target environment.

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