Add golangci-lint config and lint tasks

- Add .golangci.yml with sensible defaults for the project
- Add lint and lint:fix tasks to Taskfile
- Remove unused findGitRoot function
- Apply gofumpt formatting fixes

.golangci.yml

@@ -0,0 +1,64 @@
+version: "2"
+
+linters:
+  default: standard
+  enable:
+    - errcheck
+    - govet
+    - staticcheck
+    - unused
+    - ineffassign
+    - misspell
+    - unconvert
+    - unparam
+    - gosec
+    - prealloc
+    - revive
+    - gocritic
+    - errname
+    - errorlint
+    - nilerr
+    - bodyclose
+    - durationcheck
+    - exhaustive
+    - copyloopvar
+    - perfsprint
+    - usestdlibvars
+  disable:
+    - depguard
+    - funlen
+    - gochecknoglobals
+    - gochecknoinits
+    - lll
+    - wsl
+    - nlreturn
+    - varnamelen
+    - exhaustruct
+    - ireturn
+    - nonamedreturns
+
+  exclusions:
+    generated: lax
+    presets:
+      - comments
+      - std-error-handling
+      - common-false-positives
+    rules:
+      - path: _test\.go
+        linters:
+          - errcheck
+          - gosec
+          - dupl
+
+formatters:
+  enable:
+    - gofumpt
+    - goimports
+
+issues:
+  max-issues-per-linter: 50
+  max-same-issues: 3
+
+run:
+  timeout: 5m
+  tests: true