feat: add golangci-lint for Go code quality

- Add .golangci.yml configuration enabling govet, staticcheck, errcheck, gosimple, ineffassign, unused, gofmt, and goimports linters
- Update scripts/check.sh to run golangci-lint with go vet fallback
- Update CI workflow to install golangci-lint in both format and quality jobs
- Add golangci-lint to CONTRIBUTING.md setup instructions

Closes #266

.golangci.yml

@@ -0,0 +1,56 @@
+# golangci-lint configuration for Mole
+# https://golangci-lint.run/usage/configuration/
+
+run:
+  timeout: 5m
+  # Only lint Go code in cmd directory
+  modules-download-mode: readonly
+
+linters:
+  disable-all: true
+  enable:
+    # Default linters
+    - govet
+    - staticcheck
+    - gosimple
+    - ineffassign
+    - unused
+    # Additional useful linters
+    - errcheck
+    - gofmt
+    - goimports
+
+linters-settings:
+  govet:
+    enable-all: true
+    disable:
+      - fieldalignment # struct field alignment optimization is noisy
+  errcheck:
+    # Don't check for errors on these functions (common patterns)
+    exclude-functions:
+      - (io.Closer).Close
+      - (*os/exec.Cmd).Run
+      - (*os/exec.Cmd).Start
+  staticcheck:
+    checks: ["all"]
+  goimports:
+    local-prefixes: github.com/tw93/Mole
+
+issues:
+  # Don't limit the number of issues per linter
+  max-issues-per-linter: 0
+  max-same-issues: 0
+
+  exclude-rules:
+    # Ignore certain patterns in test files
+    - path: _test\.go
+      linters:
+        - errcheck
+    # Ignore errors from os.Remove in cleanup code
+    - text: "Error return value of `os.Remove` is not checked"
+      linters:
+        - errcheck
+    # Allow unchecked errors on deferred Close calls
+    - text: "Error return value of .*.Close"
+      linters:
+        - errcheck