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

Taskfile.yml

@@ -69,6 +69,16 @@ tasks:
     cmds:
       - go mod tidy
 
+  lint:
+    desc: Run golangci-lint
+    cmds:
+      - golangci-lint run
+
+  lint:fix:
+    desc: Run golangci-lint with auto-fix
+    cmds:
+      - golangci-lint run --fix
+
   clean:
     desc: Remove built binary
     cmds: