fix(stryker): Fix test-mutations workflow to use dotnet tool restore

.backlog/tasks/task-9 - Fix-test-mutations-workflow-to-use-dotnet-tool-restore.md

@@ -0,0 +1,49 @@
+---
+id: TASK-9
+title: Fix test-mutations workflow to use dotnet tool restore
+status: Done
+assignee:
+  - piotrzajac
+  - claude
+created_date: '2026-04-12'
+updated_date: '2026-04-12'
+labels:
+  - ci-cd
+  - fix
+dependencies:
+  - TASK-3
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+TASK-3 added `dotnet-stryker` to the repository-level tool manifest (`dotnet-tools.json` at repo root) alongside Husky and CommitLint.Net. The `.github/workflows/test-mutations.yml` workflow was not updated accordingly and still contains a dedicated "install stryker.net" step that:
+
+1. Runs `dotnet new tool-manifest` — creating a brand-new `.config/dotnet-tools.json` in the working directory, which conflicts with the repo-level manifest.
+2. Runs `dotnet tool install --local dotnet-stryker` — redundantly re-installing a tool that is already declared in the repo manifest.
+
+This step must be replaced with `dotnet tool restore`, which restores all tools (husky, commitlint.net, dotnet-stryker) from the existing manifest in a single, consistent step.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+- [x] #1 The "💾 install stryker.net" step is removed from `test-mutations.yml`
+- [x] #2 A `dotnet tool restore` step is added in its place, restoring tools from the repo-level manifest
+- [x] #3 The "👾 test mutations" step is unchanged and still invokes `dotnet tool run dotnet-stryker`
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+In `.github/workflows/test-mutations.yml`, replace the multi-line PowerShell "💾 install stryker.net" step (which called `dotnet new tool-manifest` + `dotnet tool install`) with a single step that runs `dotnet tool restore`. The subsequent mutation-testing step needs no changes as it already invokes `dotnet tool run dotnet-stryker`.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+`dotnet tool restore` resolves the manifest by walking up the directory tree from the working directory, finding `dotnet-tools.json` at the repo root. No path argument is needed.
+
+The old step used `dotnet new tool-manifest` which would create `.config/dotnet-tools.json` in the runner's working directory — a different path from the repo-root `dotnet-tools.json` — causing a manifest collision.
+<!-- SECTION:NOTES:END -->

.github/workflows/test-mutations.yml

@@ -46,18 +46,8 @@ jobs:
       uses: ./.github/actions/materialize-signing-key
       with:
         signing-key-value: ${{ secrets.SIGNING_KEY }}
-    - name: 💾 install stryker.net
-      run: |
-        $ErrorActionPreference = 'stop'
-        dotnet new tool-manifest
-        if ($LastExitCode -ne 0) {
-          throw "dotnet new failed with exit code $LastExitCode"
-        }
-
-        dotnet tool install --local dotnet-stryker
-        if ($LastExitCode -ne 0) {
-          throw "dotnet tool install failed with exit code $LastExitCode"
-        }
+    - name: 💾 restore dotnet tools
+      run: dotnet tool restore
     - name: 👾 test mutations
       working-directory: ./src
       run: |