Move CLI into a staging repository

[xmudrii]

Summary

This PR moves the CLI from github.com/kcp-dev/kcp/cli into github.com/kcp-dev/cli. The sdk repository has been already initialized: https://github.com/kcp-dev/cli

CLI is now a staged repository, similar to apimachinery, client-go, code-generator, and sdk. The source code is maintained in staging/src/github.com/kcp-dev/cli in the main kcp repo.

The reason for doing this is the same as for #3694.

This has been discussed in https://kubernetes.slack.com/archives/C09C7UP1VLM/p1761653814066419

What Type of PR Is This?

/kind api-change
(import path is changed)

/kind cleanup

Related Issue(s)

xref #3640 #3375

Release Notes

[ACTION REQUIRED] The kcp CLI has been moved from `github.com/kcp-dev/kcp/cli` to `github.com/kcp-dev/cli`. The source code is maintained in `staging/src/github.com/kcp-dev/cli` in the main kcp repo (i.e. `cli` is a staged repository). This does not effect the existing `cli` releases. The CLI users will be required to change the import paths in their Go code and `go.mod` upon upgrading the CLI.

/assign @xrstf

[xmudrii]

/test pull-kcp-test-e2e-sharded

[xmudrii]

/test pull-kcp-test-e2e-sharded

[xrstf]

/approve
/lgtm

[kcp-ci-bot]

LGTM label has been added.

Details

Git tree hash: 067deb29ac49c0c2d294093ffd4ca02e45aa2c04

[kcp-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

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[xmudrii]

/hold