refactor: switch to fork of go-keyring library

[alexec]

• Replace internal keyring wrapper with direct use of github.com/zalando/go-keyring
• Use replace directive to point to github.com/kitproj/go-keyring fork
• Update tests to clear keyring before testing missing token scenarios

[Copilot]

Pull request overview

This PR refactors the token storage mechanism by removing the internal keyring wrapper and using the github.com/zalando/go-keyring library directly, with a replace directive pointing to the github.com/kitproj/go-keyring fork. The changes simplify the codebase by eliminating platform-specific wrapper code while maintaining the same functionality through the upstream library.

Key changes:

• Removed internal internal/keyring package (wrapper, platform-specific implementations, and tests)
• Added replace directive to use github.com/kitproj/go-keyring v0.2.8 fork
• Updated tests to clear keyring state before testing missing token scenarios

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go.mod	Added replace directive to use kitproj fork; updated go-keyring version; removed unused dependencies
go.sum	Updated checksums for new fork and removed unused dependency entries
main.go	Replaced internal keyring import with direct zalando/go-keyring import
main_test.go	Added keyring.Delete call and import to ensure clean test state
mcp_test.go	Added keyring.Delete call and import to ensure clean test state
internal/keyring/*.go	Removed entire internal keyring wrapper package