Consolidate Get-Infisical* singular/plural pairs; fix PKI subscriber 404; add Get-InfisicalPkiSubscriber

BREAKING: Removed Get-InfisicalProjects, Get-InfisicalEnvironments, Get-InfisicalFolders, Get-InfisicalTags, Get-InfisicalSecrets, and Get-InfisicalCertificates. Their list behavior is now the default parameter set on the singular cmdlets; supplying the identity parameter switches to single-record retrieval. No back-compat aliases.

Fix: SignCertificateBySubscriber endpoint resolved to /api/v1/pki/subscribers/{subscriberName}/sign-certificate (was /pki/pki-subscribers and /cert-manager/pki-subscribers, both 404).

Added Get-InfisicalPkiSubscriber (List/ByName), InfisicalPkiSubscriber model, DTOs, mapper, and InfisicalPkiClient.ListPkiSubscribers/GetPkiSubscriber. MAML help refreshed for all consolidated cmdlets with 2 straight-line + 1 OrderedDictionary splat examples each. README extended with extension guide. CHANGELOG updated. 230/230 tests pass.

build.ps1

@@ -100,7 +100,6 @@ function Write-Manifest {
     CmdletsToExport      = @(
         'Connect-Infisical',
         'Disconnect-Infisical',
-        'Get-InfisicalSecrets',
         'Get-InfisicalSecret',
         'New-InfisicalSecret',
         'Update-InfisicalSecret',
@@ -108,29 +107,25 @@ function Write-Manifest {
         'Copy-InfisicalSecret',
         'ConvertTo-InfisicalSecretDictionary',
         'Export-InfisicalSecrets',
-        'Get-InfisicalProjects',
         'Get-InfisicalProject',
         'New-InfisicalProject',
         'Update-InfisicalProject',
         'Remove-InfisicalProject',
-        'Get-InfisicalEnvironments',
         'Get-InfisicalEnvironment',
         'New-InfisicalEnvironment',
         'Update-InfisicalEnvironment',
         'Remove-InfisicalEnvironment',
-        'Get-InfisicalFolders',
         'Get-InfisicalFolder',
         'New-InfisicalFolder',
         'Update-InfisicalFolder',
         'Remove-InfisicalFolder',
-        'Get-InfisicalTags',
         'Get-InfisicalTag',
         'New-InfisicalTag',
         'Update-InfisicalTag',
         'Remove-InfisicalTag',
         'Get-InfisicalCertificateAuthority',
+        'Get-InfisicalPkiSubscriber',
         'Get-InfisicalCertificate',
-        'Get-InfisicalCertificates',
         'Search-InfisicalCertificate',
         'Request-InfisicalCertificate',
         'ConvertTo-InfisicalCertificate',
@@ -201,7 +196,7 @@ if (`$cmds.Count -eq 0) {
     throw "No cmdlets were exported by the PSInfisicalAPI module."
 }
 
-`$expectedCmds = @('Connect-Infisical','Disconnect-Infisical','Get-InfisicalSecrets','Get-InfisicalSecret','New-InfisicalSecret','Update-InfisicalSecret','Remove-InfisicalSecret','Copy-InfisicalSecret','ConvertTo-InfisicalSecretDictionary','Export-InfisicalSecrets','Get-InfisicalProjects','Get-InfisicalProject','New-InfisicalProject','Update-InfisicalProject','Remove-InfisicalProject','Get-InfisicalEnvironments','Get-InfisicalEnvironment','New-InfisicalEnvironment','Update-InfisicalEnvironment','Remove-InfisicalEnvironment','Get-InfisicalFolders','Get-InfisicalFolder','New-InfisicalFolder','Update-InfisicalFolder','Remove-InfisicalFolder','Get-InfisicalTags','Get-InfisicalTag','New-InfisicalTag','Update-InfisicalTag','Remove-InfisicalTag','Get-InfisicalCertificateAuthority','Get-InfisicalCertificate','Get-InfisicalCertificates','Search-InfisicalCertificate','Request-InfisicalCertificate','ConvertTo-InfisicalCertificate','Install-InfisicalCertificate','Uninstall-InfisicalCertificate','Export-InfisicalCertificate')
+`$expectedCmds = @('Connect-Infisical','Disconnect-Infisical','Get-InfisicalSecret','New-InfisicalSecret','Update-InfisicalSecret','Remove-InfisicalSecret','Copy-InfisicalSecret','ConvertTo-InfisicalSecretDictionary','Export-InfisicalSecrets','Get-InfisicalProject','New-InfisicalProject','Update-InfisicalProject','Remove-InfisicalProject','Get-InfisicalEnvironment','New-InfisicalEnvironment','Update-InfisicalEnvironment','Remove-InfisicalEnvironment','Get-InfisicalFolder','New-InfisicalFolder','Update-InfisicalFolder','Remove-InfisicalFolder','Get-InfisicalTag','New-InfisicalTag','Update-InfisicalTag','Remove-InfisicalTag','Get-InfisicalCertificateAuthority','Get-InfisicalPkiSubscriber','Get-InfisicalCertificate','Search-InfisicalCertificate','Request-InfisicalCertificate','ConvertTo-InfisicalCertificate','Install-InfisicalCertificate','Uninstall-InfisicalCertificate','Export-InfisicalCertificate')
 foreach (`$expected in `$expectedCmds) {
     if (-not (Get-Command -Name `$expected -Module PSInfisicalAPI -ErrorAction SilentlyContinue)) {
         throw "Cmdlet not found: `$expected"