Support golangci-lint for project-wide linting

To ensure consistent code quality and maintainability, we should integrate [golangci-lint](https://github.com/golangci/golangci-lint) into the project.

## Tasks:
- Add `.golangci.yml` configuration file with standard linting rules.
- Update `Makefile` or CI workflow to include `golangci-lint run`.
- Document linting instructions in `CONTRIBUTING.md`.

## Example

This is example `.golangci.yml`:

```yaml
linters:
enable:
- errcheck
- gosimple
- govet
- ineffassign
- staticcheck
- unused
- asasalint
- asciicheck
- bidichk
- bodyclose
- contextcheck
- decorder
- dogsled
- dupword
- durationcheck
- errchkjson
- errname
- errorlint
- exhaustive
- copyloopvar
- gci
- gocheckcompilerdirectives
- gocognit
- gocritic
- gocyclo
- godot
- gofmt
- gofumpt
- goheader
- goimports
- gomoddirectives
- gomodguard
- goprintffuncname
- gosec
- gosmopolitan
- grouper
- importas
- lll
- loggercheck
- maintidx
- makezero
- misspell
- musttag
- nakedret
- nestif
- nilerr
- nilnil
- nlreturn
- nolintlint
- nosprintfhostport
- prealloc
- predeclared
- promlinter
- reassign
- revive
- rowserrcheck
- sqlclosecheck
- stylecheck
- tagalign
- tagliatelle
- testableexamples
- thelper
- tparallel
- unconvert
- unparam
- usestdlibvars
- wastedassign
- whitespace
# - wrapcheck
- zerologlint
- varnamelen

linters-settings:
gosimple:
checks: ["all"]

revive:
enable-all-rules: true
rules:
- name: "exported"
# TODO: Enable me
# all exported function should have comment.
disabled: true

# TODO: Enable me
- name: package-comments
disabled: true

- name: "add-constant"
disabled: true

- name: "line-length-limit"
disabled: true

- name: "cognitive-complexity"
disabled: true

- name: "function-length"
disabled: true

- name: "cyclomatic"
disabled: true

- name: "unchecked-type-assertion"
disabled: true

- name: max-public-structs
disabled: true

- name: "flag-parameter"
disabled: true

- name: "deep-exit"
disabled: true

- name: "get-return"
disabled: true

- name: "confusing-naming"
disabled: true

- name: "function-result-limit"
disabled: true

- name: "import-shadowing"
disabled: true

- name: redefines-builtin-id
disabled: true

- name: unhandled-error
arguments:
- "fmt.Printf"
- "fmt.Println"
- "fmt.Fprintf"
- "strings.Builder.WriteString"
- "strings.Builder.WriteRune"
- "strings.Builder.WriteByte"
- "bytes.Buffer.Write"
- "bytes.Buffer.WriteString"

gosec:
excludes:
- G304
- G204
- G115

stylecheck:
# TODO: enable ST1000 (at least one file in a package should have a package comment)
checks: ["all", "-ST1000"]

govet:
enable-all: true
disable: ["fieldalignment"]

settings:
shadow:
strict: true

predeclared:
# Comma-separated list of predeclared identifiers to not report on.
# Default: ""
ignore: "len, min, max"
# Include method names and field names (i.e., qualified names) in checks.
# Default: false
q: true

tagliatelle:
# Check the struct tag name case.
case:
use-field-name: false
rules:
json: snake
yaml: snake

nestif:
# Minimal complexity of if statements to report.
# Default: 5
min-complexity: 6

varnamelen:
ignore-names:
- ok
- ip
- no
- tt # table tests
- i # for simple loops
- j # for simple loops
- l
- h
- il
- r # reader
- w # writer

ignore-decls:
- wg sync.WaitGroup
- ts *testsuite.TestSuite
- td *testData
- ma multiaddr.Multiaddr
- db *leveldb.DB

issues:
exclude-use-default: false
exclude-rules:
- path: _test.go
linters:
- maintidx
- nestif
- gocognit

exclude:
- "shadow: declaration of \"err\" shadows"
- "builtinShadow: shadowing of predeclared identifier: min"
- "builtinShadow: shadowing of predeclared identifier: max"
- "builtinShadow: shadowing of predeclared identifier: len"
```

Start with the proposed .golangci.yml and inspect the repository's Makefile or CI workflow to choose where golangci-lint run belongs. Then update CONTRIBUTING.md with the linting instructions and verify that the configured command runs successfully across the project.

Written by the indexing model from the issue text.