fix(config): use standard user directories

[efegokdemir]

Summary

Closes #195 by using Go standard-library user directory discovery for configuration and cache paths.

Changes

• Preserve GH_CONFIG_DIR as the explicit override.
• Use os.UserConfigDir for the default configuration root.
• Use os.UserCacheDir for the default cache root.
• Preserve the existing gh and GitHub CLI subdirectory conventions and temporary-cache fallback.
• Update platform-aware tests for the standard directory locations.
• No new dependencies.

Testing

• go test ./... — passed.
• go test -race ./... — passed.
• go vet ./... — passed.
• git diff --check — passed.

Notes

No competing open PR for #195 was found before implementation. AI assistance was used; I reviewed the complete diff and am responsible for the submitted change. golangci-lint reports four pre-existing baseline issues in unrelated files: two govet inline warnings in accessibility_test.go and two staticcheck deprecations for Transport.Dial and DialTLS in http_client.go.