ci(windows): restore paths filter + document global runner scope

Re-add the paths filter (the trigger was never the problem — the runner was registered at the
wrong scope, so org-repo runs found 'no fitting runner' despite the runner showing idle). Document
in setup-windows-runner.ps1 that the registration token must be GLOBAL (Site Administration ->
Actions -> Runners), like the Linux runner. CARGO_WORKSPACE_DIR is set via GITHUB_ENV in a step
(the job-env ${{ github.workspace }} form didn't resolve, leaving it unset -> reactor build.rs
panic).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/windows.yml

@@ -12,7 +12,19 @@ name: windows
 on:
   push:
     branches: [main]
+    paths:
+      - 'crates/punktfunk-client-windows/**'
+      - 'crates/punktfunk-core/**'
+      - 'Cargo.lock'
+      - 'Cargo.toml'
+      - '.gitea/workflows/windows.yml'
   pull_request:
+    paths:
+      - 'crates/punktfunk-client-windows/**'
+      - 'crates/punktfunk-core/**'
+      - 'Cargo.lock'
+      - 'Cargo.toml'
+      - '.gitea/workflows/windows.yml'
   workflow_dispatch:
 
 jobs: