Add Gitea runner installer, proxy/SSO support, and release workflow

Scripts:
- Add scripts/Install-GiteaRunner.ps1: cross-platform installer for the
  Gitea act_runner daemon (systemd / launchd / Windows Service).
  - PowerShell 7+ runtime guard (works under irm | iex).
  - Explicit env var resolution (Process -> User -> Machine) for
    InstanceUrl and RegistrationToken with named candidates.
  - UTF-8 (no BOM) for every file write via [System.IO.File] APIs.
  - System proxy + DefaultNetworkCredentials on all web calls.
  - Optional -Labels; ServiceName/ServiceDisplayName split prevents
    systemd 'Invalid unit name' errors caused by whitespace.
  - config.yaml is always generated before the registration skip-check
    so upgrades produce a config the daemon can load.

Module:
- InfisicalHttpClient: enable UseDefaultCredentials and attach the
  system proxy with DefaultNetworkCredentials so requests work behind
  authenticated corporate proxies / SSO.
- ExportInfisicalSecretsCmdlet: make the UTF-8 (no BOM) case explicit
  in the encoding resolver.

CI/CD (.gitea/workflows/publish-psgallery.yml):
- Split into build -> release -> publish with hard `needs:` ordering
  so publish never runs unless build and release both succeed.
- Build job uploads Module/PSInfisicalAPI as an artifact.
- Release job downloads the artifact, reads the version from the
  manifest, zips the module, and creates a Gitea release tagged with
  the bare version. Release notes include version, full + short commit
  SHA, build timestamp, merged PR info, workflow run link, and any
  matching CHANGELOG.md section. Skips cleanly when the tag already
  exists.
- Publish job re-validates the downloaded manifest and runs
  Publish-Module against PSGallery using PSGALLERY_API_KEY.

src/PSInfisicalAPI/Cmdlets/ExportInfisicalSecretsCmdlet.cs

@@ -89,6 +89,7 @@ namespace PSInfisicalAPI.Cmdlets
         {
             switch (encoding)
             {
+                case InfisicalExportEncoding.UTF8: return new UTF8Encoding(false);
                 case InfisicalExportEncoding.UTF8Bom: return new UTF8Encoding(true);
                 case InfisicalExportEncoding.Unicode: return new UnicodeEncoding();
                 default: return new UTF8Encoding(false);

src/PSInfisicalAPI/Http/InfisicalHttpClient.cs

@@ -42,6 +42,14 @@ namespace PSInfisicalAPI.Http
             webRequest.UserAgent = "PSInfisicalAPI";
             webRequest.Timeout = _timeoutSeconds * 1000;
             webRequest.ReadWriteTimeout = _timeoutSeconds * 1000;
+            webRequest.UseDefaultCredentials = true;
+
+            IWebProxy systemProxy = WebRequest.GetSystemWebProxy();
+            if (systemProxy != null)
+            {
+                systemProxy.Credentials = CredentialCache.DefaultNetworkCredentials;
+                webRequest.Proxy = systemProxy;
+            }
 
             ApplyHeaders(webRequest, request.Headers);