Add contributing guide, use go.mod for CI Go version

Add CONTRIBUTING.md with development workflow and task reference.
Replace hardcoded GO_VERSION env with go-version-file: go.mod so
CI always uses the version declared in go.mod.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JAORMX

.github/workflows/ci.yaml

@@ -13,9 +13,6 @@ permissions:
   contents: read
   packages: read
 
-env:
-  GO_VERSION: "1.25"
-
 jobs:
   test:
     name: Test (${{ matrix.name }})
@@ -32,7 +29,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
           cache: true
 
       - name: Set up Task
@@ -58,7 +55,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
           cache: true
 
       - name: Run golangci-lint
@@ -78,7 +75,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
           cache: true
 
       - name: Set up Task
@@ -121,7 +118,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v6
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version-file: go.mod
           cache: true
 
       - name: Set up Task