refactor: implement dependency injection and enable parallel testing

This commit refactors the core system operations to use a manager-based
dependency injection pattern, eliminating global state and resolving
data races in the test suite.

Architecture:
- Introduced NetworkManager and NetworkOps interface in internal/network
  to abstract netlink calls.
- Introduced MountOps and FileSystem interfaces in internal/namespace
  to abstract mount and filesystem operations.
- Introduced TunnelManager in internal/wireguard to coordinate tunnel
  lifecycle using the new abstractions.
- Updated internal/cli and internal/manager to use these managers.

Testing:
- Restored t.Parallel() to unit tests in internal/network and
  internal/wireguard.
- Implemented setupParallelEnv and an enhanced mockFS in
  wireguard_unit_test.go to ensure complete test isolation.
- Added bootstrap_test.go to verify launcher preparation logic in
  internal/namespace without requiring syscall.Exec.
- Resolved data races in internal/network tests.

CLI:
- Added support for -h, --help, and -help flags for the main command.

Verification:
- Passed all tests (unit, integration, E2E).
- Verified zero data races with 'go test -race'.
- Passed golangci-lint and go vet.

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