fix(lint): resolve all 49 golangci-lint issues

[appleboy]

Summary

• Upgrade .golangci.yml to v2 with stricter linters (modernize, perfsprint, usestdlibvars, usetesting, nilnil, forbidigo, depguard, revive)
• Fix all 49 lint issues across 19 files: replace interface{} with any, use maps.Copy/strings.SplitSeq, use errors.New for static errors, use stdlib constants, use t.Chdir/t.Setenv in tests, add nolint explanations, replace fmt.Printf with proper error returns

Test plan

• make lint passes with 0 issues
• make test passes with all tests green

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the repository’s linting configuration to golangci-lint v2 with a stricter linter set, and applies code/test changes across the codebase to resolve the reported lint violations.

Changes:

• Upgraded .golangci.yml to v2 and enabled additional linters with new settings/exclusions.
• Modernized Go code to satisfy lint rules (e.g., any, maps.Copy, strings.SplitSeq, errors.New, stdlib constants).
• Improved test hygiene by using t.Setenv/t.Chdir and adding nolint explanations where required.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
version/version.go	Simplifies App var declaration to satisfy style/lint.
util/util_test.go	Switches test setup to use t.Setenv and removes error-returning setup.
util/template_test.go	Uses any + maps.Copy to populate template vars.
util/template.go	Updates template data types and function signatures to any.
util/api_key_helper_unix.go	Replaces static fmt.Errorf with errors.New for constant errors.
util/api_key_helper_test.go	Uses t.Setenv and adjusts command construction in cache-expiration test.
util/api_key_helper.go	Adds nilnil explanations and uses errors.New for static errors.
proxy/proxy.go	Uses errors.New for static errors and improves nolint:gosec rationale formatting.
provider/openai/openai.go	Uses errors.New for static “no choices” error.
provider/anthropic/anthropic.go	Removes fmt.Printf and returns formatted errors instead.
prompt/prompt.go	Adds explanation to nolint:gochecknoinits.
git/hook.go	Adds explanation to nolint:gochecknoinits.
git/git_test.go	Uses t.Chdir and simplifies error variable usage.
git/git.go	Replaces fmt.Sprintf with concatenation and adds nolint:gosec explanation.
core/transport/transport_test.go	Uses stdlib constants (http.StatusOK, http.MethodGet) and formats request creation.
cmd/version.go	Changes output printing to use color.White.
cmd/textarea.go	Uses strings.SplitSeq and adds explanation to nolint:exhaustive.
cmd/commit.go	Uses maps.Copy when merging template vars into util.Data.
.golangci.yml	Migrates to golangci-lint v2 config and enables/configures additional linters/formatters.

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