refactor(config): remove unused ConfigPath package variable

[hazelmayank]

Summary

Removes the unused package-level ConfigPath variable from pkg/config/config.go
along with the path/filepath import it was the sole user of.

Fixes #444 .

Background

pkg/config/config.go declared:

var (
    ...
    ConfigPath = filepath.Join(os.Getenv("HOME"), ".microcks-cli", "config.yaml")
)

This variable is never read anywhere in the repo. A grep for config.ConfigPath\b
returns no matches — every apparent hit resolves to the unrelated ConfigPath
field on the ClientOptions struct (opts.ConfigPath / globalClientOpts.ConfigPath).

The actual default config path the CLI uses comes from DefaultLocalConfigPath()
in pkg/config/localconfig.go, which uses os.UserHomeDir() — the
cross-platform-safe API.

If anything ever did start reading config.ConfigPath, it would silently break
on Windows: os.Getenv("HOME") returns "" on a vanilla Windows install
(Windows uses USERPROFILE), producing a broken relative path like
\.microcks-cli\config.yaml.

Changes

pkg/config/config.go:

• Removed the unused ConfigPath variable.
• Removed the path/filepath import (only used by the removed variable).

Test plan

• go build . passes.
• microcks --help and microcks context still resolve the config path
  correctly (via DefaultLocalConfigPath() in localconfig.go).
• CI passes on all platforms.